Dependency on Google Services for notifications seems odd for Proton?

I have been happily using Proton for many years now, but now I am moving away from Stock Android and to Graphene it has struck me that Proton has a big reliability on Google Services. I always felt like Proton was the big Alternative to Google and it was promoted that way (At least to me I feel like that).

But now to get Proton working I need to use Google Services or use an app that I shouldn't need to use. I currently have to use Tuta to get notifications with Proton. That feels odd and I don't think it should be the case for Proton. They should have the option to be completely separated from Google right? That should have been the goal?

Maybe I am missing something here. If so please let me know as I love Proton and love it more with all the new features and services they offer. This just seems like a glaring issue.