I think in one of his AMAs on reddit he confirmed that he never said it and no software developer during those times would say such a thing that 640K is more than enough.
Yep, when implementing a simple OS for a class, it is clear that the 640KB limitation was imposed by the hardware at the time, not the DOS. So it wouldn't even make sense for him to say it, unless it was more akin to "it's enough for us".
I don't know who said it, but it was in one book I had back in the day. That is a book from the era where 640k was something big companies might splurge on but only for systems that proved a need.