He's not. He has no wrestling background. No jiu jitsu background. He's good enough at both and yet not elite at either. For example, a wrestler in seatbelt position would never chase the RNC like he did, they would just try to pound out Luque. And a real jiu jitsu guy probably wouldn't get caught in that Darce.
I think of him as the UFC's "designated grappler", like "look, we have a grappler", and "oh look, this guy beat a grappler, he must be good against grapplers." But there are levels between where he is an where say Usman, Colby, Edwards, Burns, and also I believe Khamzat, Shavkat, and even moving to a different promotion guys like Amosov, Storley, they're on a different level. Chiesa loses the fights he should, and wins the fights he should.