Thing is... you could argue them as roughly at the same level now, but barring being taken out by injury, Fleury likely has 3-5 more solid years left. He's already passed Osgood in wins, is only 2 behind in career shutouts, is only a couple of series behind in playoff wins, has an equal number of Cups, and so on. So if you think Osgood is a serious HOF candidate now (and he regularly is considered so in HOF debates outside of HFBoards) then so is Fleury, and he's likely to become even more solidified as such by the end of his career.
I wouldn't say that Fleury is any kind of certain HOFer yet, but any of another Cup, a Vezina, or achieving 500 wins would make him so, IMO. And given this year's performance, none of those seem out of reach. Also, people tend to forget that "playing ability" is only one of four factors the HOF committee considers. Certainly it is the most heavily weighted one, but "sportsmanship, character and contributions to his or her team or teams and to the game of hockey in general" come into it, and that's Fleury in spades, especially given his role in helping to build a new team in Vegas.