MMA is still a sport in its infancy. A lot of the time, they are right. Jones, Silva, GSP, Aldo etc are some of the greatest ever and their careers have only spanned the last 12 years, they are still fighting now and at the highest level.
Also there are many facets to MMA. So striking, boxing, muay thai, grappling, wrestling, judo, BJJ, guard, top game, bottom game and so on. So to the casual, "the best mauy thai in the 145 division" or "the best judoka to ever step inside the octagon" might sound like hyperbole but it could very well be true.
I think Rogan got ahead of himself with the Ronda Rousey comment, but he wasn't totally wrong. She was finishing other female fighters inside a round and often inside a minute, so in that sense she was special talent or, at the very least, far ahead of her peers. She got rushed at the start of one fight and ended it inside 15(?) seconds with a submission (armbar) which is actually fecking insane.