There are no open email or calendar standards that handle encryption.
That would be a nice sounding argument, but actually, if you pay them, then all of a sudden calendar exporting becomes possible:
“Share calendar
Upgrade to a Mail paid plan to share your calendar.”
To confirm, click https://calendar.proton.me/u/0/ then click on the cog near your calendar on the right -> Click again the cog Settings button on the right for your calendar -> Share calendar


I ideally want full sync of course. “Well at least they have an export function” I thought, and found out it’s paid.
Anyway, I think we understand each other - just re-wording the information differently. Proton’s offering in that regard is clear.