I mean it's kinda irrelevant if it happened I wouldn't have posted it other than the fact that the dealer at the table I was playing on said he agreed it was the rule, so I wanted to make sure that isn't actually how a reasonable floor would rule.
Why do you care how a reasonable floor would rule? If you're not going to be a slow rolling douchebag you're never going to give your cards to someone and tell them to turn up one. You should also know not to muck someone else's cards if they do that to you - you have plenty of fine options mentioned here.
If you're not involved in the hand then you get some free entertainment when a slow rolling douchebag goes nuts because someone mucked his cards. Regardless of the ruling this will cause him plenty of angst which can only be fun.
I was involved in a situation like this. Major angle shooting ******* says he is beat at showdown (often does to get other player to expose) then slides his cards towards the "winner" saying - look at these. I was in the 9 and basically yelled at the dealer to muck the cards which he did. Dealer knew the angler was dirtbag. Angler was furious - called floor, said had nuts, and then got himself kicked out for the way he dealt with the floor.