The anti abuse points from their github do not sound convincing to me. There will be a high value in farming accounts to either spam or attack. What do you do when the google proxy is dosing your service?
If Google's relay just requires a Google account, there's no doubt that dummy accounts will be used to abuse the service. Apparently Private Relay sends a dynamic config of relay servers to use which they could leverage at any time to unmask you. I'm guessing Google will do it similarly.
[0] https://media.ccc.de/v/camp2023-57214-trustmerelay_investiga...