That exact quote? No, never. He said something like: current computers at the time had 64kb of RAM, so the OS was designed with a limit of 640kb, and he believed this would give them 10 years of future proofing. As it happened, that limit was reached much faster, in about 6 years.

MS-DOS didn't create that limit; the physical memory map of the 5150 did. So Microsoft (and Gates) would not have made that decision.

You are right. The quote must have been slightly different then. I'm sure about the 10 years part.