To my recollection, they're not that similar. Moulds' timed speed probably wasn't much faster, mid 4.5s or so, but otherwise, he was freakishly athletic. Instant acceleration and short area quickness to generate separation, elite contact balance, play strength, and general RAC ability, with good enough on field speed to be a vertical threat downfield. And he played with an attitude to match.
I don't think Keon rises much above average in any of those areas and maybe isn't even that in a couple. In fairness to Keon, though, that's a tough comparison for anyone to win. There is no doubt in my mind that Eric Moulds is the most talented WR the Bills have had in the 35 or so years I've been watching football. It's not particularly close, either.