I've lived in Berkeley, Oakland, and now San Francisco. Can confirm this is roughly true.
The best way to get integrated into the community is to live in a group house. If you're moving to the Bay you might have to do this anyway for financial reasons. Most of those are in Berkeley or northern Oakland.
When I lived in downtown Oakland I was already pretty far away from people (could no longer walk to my friends' houses), and I'd less often bump into folks in the community on the street.
Now I live in San Francisco. Thankfully I have access to a car, because that makes it reasonable to come to things in the East Bay. However because I don't live in a group house anymore and don't live in Berkeley, I'm definitely less plugged in to what's happening. Lots of new folks have come to Berkeley and I don't really know them. Partly this is because I'm getting old and I'm already happy with my friends and don't need to find a bunch more so am less incentivized to spend time socializing to get more friends, but also because I'm just not organically meeting new rationalists as often as I used to.
My personal experience - I live in SF and used to live in Berkley / work in Oakland -
There are more "deep rationalists" in Berkeley, Oakland; but there are enough rationalists / EAs in SF to say that a rationalist community exists here. All the big AI capabilities companies are here, and their employees are smart and familiar with lesswrong. And also the non-"rationalists" in SF are way smarter than the non-"rationalists" in Berkeley / Oakland. The baseline level of ambition / energy is way higher. So if you just want to hang out with rational people or chat about deep things then I suggest coming to SF, particularly around the Hayes Valley neighborhood.
South bay sucks though. It really suffers from brain drain. I guess David Friedman (founder of anarcho capitalism) is down there and goes to / hosts ACX meetups sometimes. I consider him the only anchor of the community down there. Almost everyone else cool has left.
To be clear, there are in-person rationality communities all over the world. For example, I live in Portland, Oregon and we have a wonderful community of about ~20 or so "regulars" and a bunch of other "dabblers".
Separately, I feel like there are diminishing returns to having more people in the community. Like, from my perspective, if there were 200 regulars in Portland instead of 20, I don't really think I'd get much if any more value. The 20 or so people are enough such that I can form a few close relationships, a handful more friendships, and have a pretty diverse[1] set of viewpoints and personalities at meetups.
I suppose the main benefits of having a much larger community are probably 1) dating and 2) professional networking. Neither are things that I personally care about, but I'm sure there are lots of people who do care about those things.
Edit: I guess there's also the benefit of rationality being "in the air" or something. For me at least my head is already swimming in it all the time -- and I actually would prefer less of it not more of it -- but I would also expect others to be different here.
Personal side note: my girlfriend is someone who appreciates and "believes in" rationality, but isn't someone who "eats and breathes" it like I am. When I was younger I thought I needed to be with someone who also "eats and breathes" it, given how important it is to me. I no longer think that though. Instead, I appreciate being with someone who nudges me more towards "turn your brain off and chill".
Such as "we should go around as a community disabling rat traps (they cause pain and suffering) and instead replacing them with something that makes them infertile because doing so will, certainly not be the most effective action we can take, but be fun and develop a stronger sense of community" and "you should claim to be nonbinary on any sort of applications even if you're not because it'll increase the chance of getting what you want". (I of course picked these examples because they're extreme and fun, not because they're at all representative.)
A cool thing about the amount of critical mass we have in Berkeley is that we can do things like have a rationalist choir! And more generally a rationalist culture, things like "songs everyone knows and will sing along with if I have a singing night", and a large set of people to find friendships and relationships in if one is picky, etc. It does have the drawback that it's easy to get stuck in a local optimum of only hanging out with rationalists, which is suboptimal in some ways. But the benefits are also pretty substantial imo.
Posting this because I recently had a conversation that went like this:
Friend: Hey, you used to live in SF. Is there any rationalist stuff actually happening in San Francisco? There don't seem to be many events, or even that many aspiring rationalists living here. What's up with that? [Paraphrased. I've had similar versions of this conversation more than once.]
Me: Something we realized living there is that SF actually suffers the same brain drain as most other cities, because everyone just goes to Berkeley/Oakland.
The same way people move from the East Coast or elsewhere to Berkeley, they move from the rest of the Bay Area to Berkeley. Actually, they do it even more, because moving to Berkeley is easier when you already live pretty close by.
And you don't figure this out until you move there, because people who live outside the Bay Area think of it as being all the same place. But the 45 minute train ride really matters when it comes to events and socializing, as it turns out.
Friend: That sounds so inconvenient for people who have jobs in the city or South Bay!
Me: Sure is! I don't have a super-solid answer for this, except that 1) Lots of people actually just do awful, awful commutes, because having a real, in-person community is that valuable to them, as bad as commuting is. 2) A surprising fraction of the community works at rationalist/rationalist-adjacent nonprofits, most of which are actually located in the East Bay. Plus, 3) in a post-COVID world, more people can work remote or partly remote. So you can choose to live where your community is... which is Berkeley... even though it is crazy expensive.
I don't actually live in the Bay Area anymore, so I don't have the most up-to-date information on where events are happening and things. But it seems from what I hear from folks still there that it's still broadly true that East Bay is where things are happening, and other parts of the area have much less of the community.
If you're thinking about moving to the Bay in part for the rationality community, take this into account!