he doesn't get slammed nearly as bad if the body triangle isn't in. and by "let go of the body triangle" that doesn't mean he keeps his legs hooked in, either. you let go of the position. Usman isn't GRABBING Khamzat and slamming him. Khamzat is HOLDING on to Usman. If he gets slammed, that's on his dumbass for not letting go of the submission. Khamzat has plenty of opportunities to let go or change position before he gets slammed with Usman when he dives like that.
If a fighter is fully in control of avoiding getting slammed by simply letting go of a position I don't see why it should be illegal, especially a position like an RNC where it's easy to just let go. A triangle is a bit different so there's a decent argument there.