In other words, the wording of thier response indicates to me personally there's probably written evidence of the whole bonus thing
I hope so, for their sake, because the world's a shithole in my experience, and most bosses won't give you any more if they don't have to. They could be laughing all the way to the bank while you're doing all the work, they won't care. And they'll sleep well at night, too.
If they got something down on paper that says they're due, like a contract, then it's a totally different ball game. In which case, it was stupid of Activision to not have paid up, because now they're just going to end up having to pay more.