It connects to proprietary services which makes it not an option. The same situation for Tuta Mail. We can talk after they publish their server source code. Proton is also not an option for the same reasons.
Our team created a custom notification solution and managed to implement it in an energy-efficient way, an alternative to Google’s FCM that lets us fully bypass Google’s infrastructure.
FFS, it's a calendar. It has no business connecting anywhere.
Its a similar problem with Proton for me. Proton doesn't even let you sync to 3rd party cal app on the free tier.
If you're going to paywall basic functionality, that just prevents me from trying it for a while and eventually become so tied to it, it's difficult to break free. Google had the right plan from the get-go.