You know, the more I think about this, the entire premise of the MSJ just hasn't made sense from the beginning. With as much DD as Susman Godfrey did on this case, surely they would have caught the date discrepancy from the very beginning. This is what baffled me, and others, when the MSJ was filed.
What if SG did catch it, but left it open as a trap? ATVI files the expected MSJ, USPTO corrects it quickly, and ATVI is left completely boxed in. It prevents ATVI from pursuing other angles in a MSJ, at least in the short term.
I am not a lawyer and I am speculating wildly, but it sounds like good strategy to me.