Summary: title + LW seems to be already coming from a culture like that, I just want to make it conscious. Also, it is true for individuals and true for groups like nations.
Why too much "jock" is bad
Anti-intellectualism, black-and-white tribalism, impulsivity etc. Social experiment: if you know smart people in/from Latin America ask them about what they dislike there, I think they will tell things along the lines of too much machismo in the culture not being a good environment for intellectualism.
Why too much "geek" is bad
It turns people and cultures into "Markos Sophisticus".
I was shaped largely by "Sophisticos". Too much Vienna and too much Budapest in me, thankfully some Birmingham in me too (Anglo cultures healthily tend away from this) but not enough. It was kind of shocking or me to see LW using intelligence not for intellectual masturbation but for solving problems, getting things done, making actionable things and actioning on them.
This has a bit of a "jock" in it, a bit of that attitude that: problem? Hit it over the head with a suitable club. Problem solved. Next problem. This "checkmark-checkmark-checkmark" attitude.
All this challenging real world problems head-on is a bit of a "jock" thing but you probably don't realize it if you are socialized to Silicon Valley, Paul Graham kinds of cultures. In these cases all you notice is that you are smarter, geekier than the "jocks" around you. But actually you are "jockier" than some of the more "geekier" cultures around the world e.g. the kind of "Sophisticos" attitudes that are all too common here in the Mitteleuropa - read some Stefan Zweig. Or rather not, he will bore you to death, you may as well take my word for it that I was shaped into using intelligence as simply something to bask in it, in my sophisticatedness, and not for making the world or my life better. It was a very novel idea for me that ideas should be actionable. I think there is also a power problem : things that needed to be actioned were usually prescribed by others and usually not by very smart people.
I mean I know a lot of "Sophisticus" people who would dislike that above mentioned essay. Darwin i.e. dying if you are a wrong as an argument, nature being cruel? "How impossibly crude and barbarous to reason like that in a debate between intellectuals!" That kind of stuff. The "it is a tough world out there" message of the essay is clearly a bit "jocky". Some people would even consider that essay borderline "reactionary" for its tough-world "assumption".
And that is how too much geek hurts you. Of course you want a lot of geek. You want to be smart and rational, knowledgeable and learned. But you also need that bit of jock in you who actually wants to hit problems over the head with all this and check-off to-dos and changes things.
I suspect that was part of our problems in Europe around 1920-1930 that led to WW2! You observe the era and you find various kinds of fascists e.g. in Italy or even Action Française glorifying "direct action" and having an ultra-masculinist outlook. But why? Or at least, why did this had appeal even to people who were not, at least initially not, evil? Well, at least partially its appeal is explained by it being a reaction against an era of intellectual elites being all too talky and never being able to decide something, make an action plan, and do it. Too many smart words and not enough strength to actually do something to improve things. So other people went the other extreme and began worshipping direct action, strength and violence.
Of course this model does not even begin to try to predict all that happened in that era, I am just saying when non-intellectual people start worshipping direct action and strength, that is a canary in a coalmine signalling intellectuals are being too geeky, ineffectual, and not actually proposing actionable ideas nor insisting enough on following them through.
The "too geek" has smart but unactionable ideas, the "too jock" just takes direct action without much thinking, "smart, rational, actionable" ideas are a balance.
Again, if you are a Silicon Valley type you are probably too much used to this balance and assume you are being "geeky" when in fact you are actually being a healthy balance of "geeky" and "jocky".
E.g. the term "hacker" a lot of Valley people identify with originally came from someone making furniture with an axe - a definitely crude and unsophisticated, not too classy profession, but very practical and someone who is not too upset from the fact the world out there is tough!
Why does this matter?
To improve the world, try to move things toward this balance everywhere. At the very least, move your friends. Move your jocky friends towards gathering knowledge before acting, move your geeky friends towards focusing on actionable knowledge not intellectual masturbation.
And, if possible, move whole nations.
(Also, I realized I am using very stereotypcially and narrowly male terminology here. It all boils down to thinking without acting vs. acting without thinking vs. both, so thinking and acting, but this is a very textbook male view of things. My wife rather likes to move within the triangle of thinking - feeling - speaking, so adds the feeling component which my geek-jock axis completely ignores, and focuses more on speaking than physically doing. However, it is also true that speaking and doing should not be contrasted much, speaking is a form of doing. I am just saying this to signal I am not entirely unaware of my viewpoint here being a bit too narrowly male and not really inclusive enough.)
What am I even trying to say?
Reading what I wrote so far sounds like I am saying nothing. Actionable ideas that pay rent etc. have always been a core idea on LW. I am simply saying it is not just being a better geek, it is actually being a bit of a jocky geek, I am saying geeks to some extent picked it up from jocks. Possibly, in places where jocks do not hate geeks much but tend to have some respect of shrewd people, jocks push geeks toward practical problems and geeks accept this, or geeks, not hating jocks that much, observe and learn the practicality.
I am simply saying that his kind of instrumental rationality is a crossover between being a philosopher and being a plumber and not an entirely new dimension of things. There are clearly a lot of cultures where plumbers should be more like philosophers and scientists. But there are also a lot of cultures or even individuals even in Anglo cultures who are too much of a philosopher, and need being more of a plumber.
My point is simply that instrumental rationality is not a new kind of geekery but a way of learning from non-geeks: I suspect that in Silicon Valley type cultures this learning happens automatically and subconsciously, maybe it happened so long ago that now geeks learn it from each other, not directly from non-geeks. But this learning needs exported.