At what point is it, though? From the Client or when it hits the server. If it is the later, we can’t know if three letter agencies intercept the traffic before it gets anonymized.
Furthermore, it is often possible to de-anonymize data especially if you have an extensive knowledge of users and their data such as google. But even then, you can also de-anonymize data
The client can’t anonymise the IP as it connects to the Google server which then will implicitly know it. What the flag does is tell the server to anonymise it from that point onwards. From my original link:
When a customer of Analytics requests IP address anonymization, Analytics anonymizes the address as soon as technically feasible at the earliest possible stage of the collection network. The IP anonymization feature in Analytics sets the last octet of IPv4 user IP addresses and the last 80 bits of IPv6 addresses to zeros in memory shortly after being sent to the Analytics Collection Network. The full IP address is never written to disk in this case.
Even if they could, they wouldn't be able to find it.