Dr. Donald E. Knuth seems to be doing well --- made enough to at least include a mention of what he did with his royalty checks in an index entry in one of his books (royalties, use of points to a page with a graph which resembles the pipe organ which he had installed in his home).
There are always outliers (that's the whole point).
All he was saying is that when the market for technical books was new (because that was really the only way to get to speed on something) you didn't need to have Knuth-levels of knowledge and writing skill (Knuth is both incredibly knowledgeable and an exceptionally good writer) to make a decent living.
