I was thinking simply, "too little, too late" but then realized this could be quite interesting. Uber's model in most domains is to push hard over the line in the hopes of moving that line. More charitably this could be called "ask for forgiveness rather than permission". So canning Levendowski could be seen as simply a case of this. But Google's suit is against Uber, not Levendowski, who could now spill beans on all…
Google's suit is against Uber, not Levendowski, because Uber has the capital to go after. I Am Not An IP Lawyer, but I'd guess that if Levendowski had substantial assets, he'd be named a party in the complaint.
My guess is that this firing is mainly a move to improve Uber's political capital "Look we fired the bad apple". I'd assume Uber has probably figured out how to tie his interests into cooperating with him (to the detriment of exposing IP theft) though maybe granting him equity or some sort of financial incentive.
Uber: Hey, we're really sorry that things didn't work out but "here's some equity for the time that you worked here" oh and by the way here's the estimated value of that equity if we win the lawsuit or don't win the law suit.