N1b No, that's not what he meant. He literally said you wouldn't get push notifications if you choose a different apk source, which is not how it works. It's just a case of a troll writing things to feel smart, without actually knowing what they're talking about. No matter where you get your Proton Mail apk from, it won't change a thing about its push notification functionality. It doesn't matter, though, I'll just ignore it from now on.
I don't think the choice of using Obtainium is obvious at all. If you read the GrapheneOS twitter account, for example, you often see them talk about how the Play Store offers a more secure app installing and updating process in general, compared to stuff like Obtainium. The only other app store I ever saw recommended from GOS staff was Accrescent, which unfortunately is not useful for me because it doesn't have the apps I want.
I think I'll just stick to the Play Store for any installing and updating of apps whenever possible, despite delays.