Mayweather will cover up when/if he's in trouble, and he'll also hold. He's a defensive counter-puncher, and you're right about his general guard position. His high (vertical) right guard protects his head and keeps him ready to reply. For that matter, telling a tcc person (facing a boxer or anyone) to keep his hands up usefully seems like common sense. Part of the reason is that boxers train to exploit that defect. By keeping his lead arm low and using his shoulder to deflect, he is able to protect his mid-section and use his left as a follow up.
Otoh, don't forget his conditioning, speed, footwork, and head movement . Have you watched his fights with Canelo and Cotto?