Can you cite this in some way? Given he's shown the competence to write and typeset an impressive series of books, I find this claim pretty hard to believe.

https://www-cs-faculty.stanford.edu/~knuth/programs/.fvwm2rc

Here is his fvwm rc. Given that it's fully documented, I will walk back my assumption that he can barely open a terminal. I researched it a bit and recalled an interview where he said something like "all I use X windows for is is to open a terminal in FVWM", so he clearly can customize it, but he prefers a minimalist setup.