Interesting. I always interpreted it as, "Don't sweat the details yet- you don't even know if anybody wants what you are building. Find that out first."
The mindset of "fail fast" wasn't a thing when Knuth wrote that statement.
This isn't "fail" so much as it is acknowledging that neither you nor your customers will know what they like until they have something to play with.
My point was that Knuth wasn't considering that possibility when he wrote the quote. In those times it was assumed that if you were writing software, there was a darn good reason for it. The idea that you might write software that doesn't fulfill a compelling need is a rather modern invention.
Except that optimizing cold code is writing software that doesn't fulfill a compelling need.