While what you said may be true simply by the virtue of graduate level textbooks being so dense, I think GP wanted to imply that "just reading a moderate amount

While what you said may be true simply by the virtue of graduate level textbooks being so dense, I think GP wanted to imply that "just reading a moderate amount of mathematics" isn't sufficient to get anywhere. I would say that 3-4 entire graduate level textbooks (which you wouldn't understand anyway without having done the undergraduate stuff beforehand) is much more than "a moderate amount".