This group is not 2000 people. There are two (?) NLRB-certified bargaining units: 41 who work at Cognizant (the unit at issue in this case) and 119 who work at Accenture (although apparently 80 of those were later laid off).

The remainder of AWU is people who have put their name on a spreadsheet but have not voted in a union via an NLRB election. The only way to have the legal right to force an employer to bargain with you is to have an NLRB election (or the employer voluntarily recognizes the union).

Google has a legal duty to negotiate in good faith with the 41 Cognizant employees now that the NLRB has found them to be a joint employer. That's true regardless of how small the bargaining unit is. The NLRB found that Google controls essential features of these workers' jobs, which means Google needs to appear at the bargaining table. However, Google is allowed to appeal that finding before it takes effect, which will likely take at least a year in the Federal courts.