Bubba, the thing here is that the whole exchange between Punk and MJF doesn't really "endorse" the company or WM. MJF, kayfabe-wise, thinks he's above Punk and AEW as a whole. He's doing them a favor by being there, and he doesn't seem properly valued. Him going to WWE and main-eventing Mania (lololol) would be a middle finger to AEW and to Punk, who never was able to do that. It fits with his whole schtick of "being better than you".
When Punk retorts, it just exposes MJF kinda being a fraud. Main eventing night four and then quickly being released means he ain't shit, not to his eyes, nor to the eyes of the company he wants to go just to spite AEW. It works.