"Our policy only impacted cold calling, and we continued to recruit from these companies through LinkedIn, job fairs, employee referrals, or when candidates approached Google directly. In fact, we hired hundreds of employees from the companies involved during this time period."

It doesn't sound like it was very effective. Cartels in general tend to be unstable, as members have powerful incentives to cheat on the agreement.

It's hard to say if this agreement actually reduced salaries in any significant way, as there were plenty of other companies in the area who were not part of the cartel.