Difference being Direct x isn't exclusive to either brand, and secondly Microsoft dragged their heels on working on a new version because they're more console oriented these days.
As for mantle and the delay its pretty obvious that was down to dice, battlefield 4 was broken from launch and its only recently they've made larger changes to get the game working properly.
I think GM's point is when mantle is mentioned vs dx 12 its said Mantle is here now DX12 isnt so cant be compared,, Yet when gsync is mentioned Freesync is on the way and will be better,cheaper without any of us knowing this and in stark contradiction to his DX example
DX might not be exclusive but then neither is freesync and mantle/gysync both are so imo its a valid comparison