I think it makes sense from the angle of Musk wanting attention and people mindlessly praising him and his "brilliant decisions." A savvy marketing trick where he pairs up with another company means he has to share the glory, but a restaurant for his hardcore fans means everyone involved is going to just be saying how great and brilliant it all is, right up until the place goes bankrupt 6 months from now. I don't think Musk is capable of making a proper business decision at this point, he's too surrounded by Yes-men.

Additionally, this could also be a kind of attempt at a sort of soft cult building thing or future planning for the collapse of the US. Where he tries to make his biggest fans wholly reliant on his "brand" for everything in their lives, if so, we'll probably see a Tesla/"X" branded streaming service, news and basically anything else in order for him to eventually try and carve out a little fiefdom of loyal subjects for when the US collapses into Warlordism. That would require a lot of forethought from him though. It's probably more likely that he just thought he was great and amazing and could make the world's greatest restaurant despite probably not even knowing how to boil water.