Correct, in this case Blizzard would have to release a mount NFT whose ownership their code checked. There must be a first party NFT market for the second party one to make sense.

The system as described doesnt 'break down' if that transfer doesnt relay to in game, it never launched whatsoever.

I dont know what would be in it for developers to tie into such a system - unregulated video game markets are notorious for the scams they breed (don't believe me? Meet me in Thais depot for some fast hands), so ceding control so that a secondary market can spring up sounds.. counterproductive.