This is the most coherent version of the story I think. You knew it had to be Elon’s idea because Trump would never think of a speed limit cap or know it could be a thing, but if Trump didn’t know about it ahead of time he and his henchmen wouldn’t have spent all week pretending it didn’t happen.
Musk is a direct threat to national security, whether he conspired with Trump or not—it’s not clear which is worse, and the answer being unclear is bad in itself.