Actually this is only partly true.

Here's a compressed timeline of books and other works of Knuth:

1962: Starts writing TAOCP.
1968, 1969, 1973: Volume 1, 1st edition.
1969: Volume 2, 1st edition.
1973: Volume 3, 1st edition.
1974: Turing Award.
1974: Volume 1, 2nd edition.
1974: Surreal Numbers (written in 6 days “and on the 7th day he rested”).
1976: Mariages Stables et leurs relations avec d'autres problèmes combinatoires.

1977 February: Receives galley of Volume 2 2nd edition (publishers have switched from hot-metal typesetting (Monotype) to phototypesetting, is dissatisfied with typography and excited by the possibility of getting it right with digital typesetting). Plans to have galleys ready in the summer :-)
1978 January: Gives Gibbs lecture (“Mathematical Typography”) on his ongoing research, unexpectedly many people are excited and want to use TeX.
1978: Finishes first version of TeX (aka TeX78, in SAIL)
1980: Starts work on rewrite of TeX (so that it can be used outside SAIL).
1981: Mathematics for the Analysis of Algorithms.
1981: Volume 2, 2nd edition. Realizes fonts still look bad on paper, starts getting more font design feedback from the masters.
1982: Finishes TeX, starts rewrite of METAFONT.
1984: Finishes METAFONT.

1986: Volumes ABCDE of Computers and Typesetting
1990: Announces end of his work on TeX and METAFONT (will only fix major bugs; others can continue writing other programs)

1992, 1996, 1999, 2000, 2003, 2003, 2010, 2011, 2011: Eight (plus one) volumes of collected papers.

1989: Mathematical Writing (lecture notes)
1990: 3:16 Bible Texts Illuminated (stratified random sampling of the Bible, with illustrations by many designers).
1992: Axioms and Hulls.
1993: CWEB.
1993: The Stanford GraphBase (some toy programs in CWEB).
1994: Concrete Mathematics.

1997: Volume 1, 3rd edition
1997: Volume 2, 3rd edition
1998: Volume 3, 3rd edition
1999: MMIXware

2001: Things a Computer Scientist Rarely Talks About (based on 1999 lectures at MIT on religion)

2005: Volume 4 Fasc 2 (part of Volume 4A later)
2005: Volume 4 Fasc 3 (part of Volume 4A later)
2006: Volume 4 Fasc 4 (part of Volume 4A later)
2008: Volume 4 Fasc 0 (part of Volume 4A later)
2009: Volume 4 Fasc 1 (part of Volume 4A later)
2011: Volume 4A.

2015: Volume 4, Fasc 6 (middle third of Volume 4B)
2018: Fantasia Apocalyptica Illustrated
2019/2020: Volume 4, Fasc 5 (first third of Volume 4B)

Planned:
????: Volume 4, Fasc 7 and 8 (last third of Volume 4B)
2025 (yeah right): Volume 5

So although he definitely badly underestimated how much time the typography project would take (he thought it would be done in a summer! then again, he also thought in 1962 that he'd be done with TAOCP in a year or two), and although it's technically correct that 38 years passed between 1973 (1st edition of Volume 3) and 2011 (Volume 4A), he wasn't spending all of it on typography; much of it was spent on other projects and on producing new editions (which in his case are a lot of work!) of earlier TAOCP volumes and other books. And even during the period 1977-1989 when he was working primarily on typography, he somehow managed to publish about 50 papers (P77 to P125), 60 other papers (Q48 to Q110), and 20 reports (R35 to R55): https://cs.stanford.edu/~knuth/vita.pdf (I really wish someone would parse his vita.tex from https://cs.stanford.edu/~knuth/vita.html, make it machine-readable and put it on a nice webpage sortable by year and topic and listing newer editions etc).