It's funny, I'm about 9 months behind on the Joe Rogan Experience podcast and last year they were discussing Lesnar returning. They said he was in talks but opted out as he wasn't 100% committed, and unless you're 100%, you don't have any business fighting MMA elite. Joe was glad he didn't come back for a payday.
The money must be too good for UFC 200 for him to resist though. I highly doubt this is a long term thing for Lesnar. Probably trying to end his MMA "career" with a win. Honestly I don't really care about the fight as it's a spectacle more than a serious match.