I haven't read Tim's books but this seems like a reason to not like his work. This isn't science. Now, if he said body weight from water etc. Which he could have for all I know.
When I first started training Olympic weight lifting seriously I gained nearly fifteen pounds of muscle in six months (along with five pounds of fat; even vain bodybuilders who are obsessed with body fat have to alternate bulking and cutting cycles because it's very hard to gain a lot of muscle while keeping off the fat) and my coach has replicated that routinely starting with relatively weak beginners. About six months into a program like that, your efforts start to yield diminishing returns. This is true for total performance as well: when you're starting out, for the first six to eight months, it's easy to work out 3-4 times a week, do heavy squats every time, adding 5 kg to your old record with every single session, but once you get to about 2x body weight (around 150 kg squats in my case) on the bar you not only can't add things as fast anymore, but you have to totally change the nature of the program, and this is despite the fact that 5 kg represents an increasingly smaller percentage of your maximum capacity, so you might naively think it would get easier, not harder, over time to keep up a constant rate.