Ngannou is not even a boxer, Joshua is currently in some kind of crisis, but that does not mean that Ngannou can be placed on the same level as Joshua in terms of boxing...
It's not just that he's an MMA guy. I've seen his fights and he loads up his punches from roughly around Cameroon. Sure if they land you're in a boat of trouble but a guy like Fury will have time to sing a song, flirt with the girls in the front row and still slip those shots with about a minute to spare. Now, sure, if Fury decides to turn it into a phonebooth war then Ngannou will have a chance but Fury never struck me as stupid.
Name me one thing Ngannou can do that AJ can`t, MMA fighters don`t throw as many punches as boxers, how many punches has Ngannou evaded?
He'd leg kick AJ's head off then grapple his limp body into a bodybuilder size bodybag. Then the boxing board will disqualify and ban him for using illegal blows.