Or perhaps we should be asking: What are all the other NSF-funded scientists doing on their bathroom breaks? Knuth's output has clearly been accounted for.

This paper was published in the American Mathematical Monthly, a journal which is mainly known for its expository articles, not necessarily for original research.

For that purpose (exposition) this paper does its job admirably. Whether it's useful research is not really important in evaluating whether it was worthy of its funding.