It is but Google doesn't directly employ these workers. They're subcontracted through another firm. Don't get me wrong I hope they get everything they're demanding, but I'm not sure Google has any legal obligation, or even the ability to make changes to their contract.
The employees are not contractors, but it looks like they work for a firm which has been hired by Google. Reading a little further it does look like NLRB has found that Google is considered a joint employer and would be obligated to bargain. Which they're not doing, I guess hoping for an appeal to go their way.