Google was like "Hey, FEC, we're planning to do this thing; is it illegal? Our lawyers want to know if this counts as a campaign contribution before we start doing it and accidentally break the law". The FEC considered it and replied with a public letter stating that, in their opinion, this isn't a crime.

Final AO: https://www.fec.gov/files/legal/aos/2022-14/2022-14.pdf

Any other email provider could do the same now of course: it's just an opinion about the law, not a rule specific to Google.

Google also could have gone ahead and just done it, of course, but having the AO in hand puts the lawyers at ease.

I read it as Google wanting political parties to explicitly register themselves and to use proper email authentication. Then if they start enforcing it, they can't be blamed being biased (again!).

It's not that they are asking if they may let political spam trough, they want accountability from senders and freedom to mark it as spam based on "user consent". Maybe I'm looking at this too positively but to me it sounds very convenient to Google's spam filters.