He'd probably surprise a few... But then a gatekeeper with a decent chin and a bit of power sometimes can - and that's realistically where he'd have been likely to sit. It wouldn't surprise me if he beat Wilder, for example. He'd have always struggled with the real top guys, but he had decent durability on his side and could punch. He might have some luck with someone like Hrgovic who wouldn't have any chance of stopping him, and who would've given him a chance on the cards thanks to his poor gas tank... But then, in my book, Hrgovic hasn't really proved he's anything beyond gatekeeper, either.
I could see Ngannou knocking Joyce out if he goes all out for the KO early but obviously thats still a tall order, as Joyce's defence is so poor and his punch resistance is declining. Thats about it really, and Joyce isn't even top 10.
Dillian Whyte, Wilder, Joyce, and possibly Bakole. Then again if fully motivated they could all make very quick work of him as well.