Knuth doesn't mean "art" as in "fine art to be admired" but rather "art" as in a description of something about which it is not possible to be purely scientific about.
> To summarize: We have seen that computer programming is an art, because it applies accumulated knowledge to the world, because it requires skill and ingenuity, and especially because it produces objects of beauty.
(See also, from an earlier paragraph “When I speak about computer programming as an art, I am thinking primarily of it as an art form, in an aesthetic sense.” — though the summary is a better reflection of his views.)
> To summarize: We have seen that computer programming is an art, because it applies accumulated knowledge to the world, because it requires skill and ingenuity, and especially because it produces objects of beauty.
(See also, from an earlier paragraph “When I speak about computer programming as an art, I am thinking primarily of it as an art form, in an aesthetic sense.” — though the summary is a better reflection of his views.)