Google is sitting on the "but they're contractors!" angle because it makes it easier for them.
Why?
Because once the union does collective bargaining with their actual employer, Cognizant, the company will have almost no recourse but to increase fees to Google for the contract work.
Once this happens, Google just says "Oops, you're shit out of luck" and then hires a whole new company of contracted workers for the same work, for cheaper.
Google purposefully uses this type of structure to ensure they never have to pay more, even when collective bargaining with unions does happen. Because then they can just shitcan the whole company and claim costs were too high. They certainly won't break their contract, but you can bet your ass when time comes to renew it, Google will have found someone new to take their place.