Actually, I agree with this. I was talking to some friends about this, and we all agreed that Phelps probably isn't the "greatest Olympian ever" simply because athletes in other sports can't compete in as many events based on their one skill. (However, we still said he was a machine, and didn't discuss it beyond that).
Weightlifters will never get 8 gold medals. Same with table tennis champions, basketball, or gymnasts. Even a sport like rowing, which has quite a few events for athletes to compete in, couldn't get more than 3 or 4 gold medals if they competed in absolutely everything.