No Man's Land is where you suffer unnecessarily, make lots of noise and ultimately accomplish nothing.
I agree that your statement is a fair take-away for well-trained athletes, but I think it ignores the nuances of the book's advice for beginners.
Here's what the book has to say:
For the well-trained… [zone 2] carries with it a burden of fatigue unworthy of its benefits. Zone 1 will therefore make up the bulk of your base training if you have a very strong background of aerobic endurance training.
However, for those with little aerobic training history (referred to as Aerobic Deficiency Syndrome) this zone [zone 2] can be used for virtually all aerobic base training. The reason is that the pace at this intensity will be quite slow and therefor [sic] there will be little accumulation of neuromuscular fatigue and this effort level won’t even feel like training.
—Training for the New Alpinism, first edition, page 59
TL;DR
If you're an ultraendurance athlete then you should spend three quarters of your time just below your Aerobic Threshold and most of the rest just below your Anaerobic Threshold. Everyone benefits from a little strength training.
What it Takes to Climb a Mountain
There are three components to climbing a mountain. You need the right skills, the right equipment and the right body. Training for the New Alpinism: A Manual for the Climber as Athlete by Steve House and Scott Johnston is about how to build the optimal body for alpine mountaineering.
Training for the New Alpinism is among my favorite books on athletic training. It has great information on subjects from altitude acclimation to "whether Peanut M&M's are in fact nature's most perfect food", but my favorite chapters are endurance conditioning[1] and physiology[2].
Training for the New Alpinism is concerned with an athlete's ability to exert force. In this context, physical conditioning has four components.
Peak Speed Doesn't Matter
Speed is measured in fractions of a second. Mountain climbs are measured in days.
Muscles are Heavy
Increasing your strength has two phases. At first, your body will increase muscle recruitment. This is a neurological adaptation. It adds no weight. Muscle recruitment strictly adds to your endurance because it spreads your exercise load across more muscle fibers who take turns contracting which increases how long it takes to exhaust each individual muscle fiber.
A minimum muscular strength is necessary to climb a particular mountain. If you have good muscle recruitment then additional strength gains are counterproductive to mountain climbing because increasing strength (beyond muscle recruitment) increases your weight. Carrying extra weight makes it hard to climb mountains. A specialized mountaineer builds no more muscle than he or she needs.
Muscular strength is a precondition to muscular endurance. Alpine mountaineers build a little bit of muscle mass and then extract as as much muscular endurance as they can out of it. According to the authors, "You'll do it via very sport-specific movements in workouts using up to hundreds of repetitions."
Aerobic Endurance Zones
Endurance is the most important attribute of an alpine mountaineer. The authors quantify effort by bucketing heart rate into six zones. The heart rate (HR) percentages are rough guides. What really matters is the state changes.
Recovery Zone (<55% of max HR)
Light exercise speeds recovery when compared to sedentary activity "because aerobic enzymes and hormones…actually improve the rebuilding process of the structures damaged during harder training" but the exercise isn't hard enough to actually damage the structures.
If you're exercising so hard that you're no longer recovering that means you have entered Zone 1.
Zone 1: Basic Endurance (55-75% of max HR)
I think of Zone 1 as the pace you can sustain all day.
Zone 1 the most important attribute of a mountaineer because climbing a mountain takes multiple days. Mountaineers should spend most of their time training in Zone 1 because the way to get better at Zone 1 endurance is to train in Zone 1. Zone 1 is very light. To get the most out of this training, mountaineers compensate with volume. Optimal training requires an average of at least two hours of Zone 1 training per day, everyday.
My preferred method of Zone 1 training is to ride my bicycle to a mountain and then climb the mountain. Zone 1 feels good. It's one reason why bicyclists smile so much. (The other reason is Zone 3.)
The Aerobic Threshold (AeT) describes the boundary between Zone 1 and Zone 2. Recall from the introduction that most of your training should take place right below the AeT.
Zone 2: No Man's Land (75-80% of max HR)
Zone 2 is too easy to be useful and too hard to justify the effort. Amateur athletes frequently train in Zone 2 because it's what they think training is supposed to feel like even though Zone 2 is precisely the opposite. No Man's Land is where you suffer unnecessarily, make lots of noise and ultimately accomplish nothing.
Zone 3: Upper Aerobic Training (80-90% of max HR)
Zone 3 provides uses half aerobic and half anaerobic energy.
Zone 3 is not peak speed.
The boundary between Zone 3 and Zone 4 can be called the Anaerobic Threshold (AnT) or the Lactate Threshold (LT). Be careful not to confuse the AnT with the AeT. The AeT marks the boundary between Zone 1 and Zone 2. The AnT marks the boundary between Zone 3 and Zone 4.
Zone 4: The Anaerobic Zone (90-95% of max HR)
Zone 4 has costs and benefits. The benefit is it increases your VO₂ max. The disadvantage of Zone 4 training is it trains your anaerobic energy pathways more than your aerobic energy pathways. A mountaineer's Zone 4 training must be balanced against a large aerobic base developed in Zone 3 and (mostly) Zone 1.
That "10 percent" is for athletes like marathon runners. Alpine mountaineering makes marathon running look like a sprint. VO₂ max matters little to alpine mountaineers whose event durations are measured in days and weeks rather than hours.
Alpine mountaineers do not need Zone 4 training.
Zone 5
Zone 5 is strength training. It is purely anaerobic. A little bit of Zone 5 training first increases muscle recruitment. Additional Zone 5 training builds muscle. Muscle recruitment increases endurance without building muscle. In this way, at least a little bit of strength training is useful to all alpine mountaineers, including those who do purely nontechnical endurance-oriented climbs like Mt Rainier.
The information on endurance training is so good it was used by lots of athletes in adjacent endurance sports. In response to all the positive feedback, the authors wrote a second book for mountain runners and ski mountaineers. ↩︎
This article summarizes the the endurance section. If you want to understand the physiology underlying these training methods, I recommend reading the book. ↩︎