This idea that MMA promotions somehow make fighters better rather than the other way around is stupid and I don't know why it never dies other than propaganda and casual fans repeating it.
Eddie Alvarez wasn't a UFC fighter came into the UFC and won a belt, Justin Gaethje wasn't a UFC fighter came into the UFC beat multiple top 10 fighters and fought for the belt, Melendez wasn't a UFC fighter fought Henderson for the belt and about half the people watching that fight thought he won. Chandler KOed a top 6 guy in a round and nearly KOed the current champion in their fight.
Shogun won the UFC belt after major knee surgeries, Zombie Big Nog won an interim HW title. Pettis and Henderson were from WEC both won gold in the UFC. Just because a top level fighter isn't in the UFC doesn't mean they aren't a top level fighter.