I don't think that's entirely correct? Calendars are part of the OS:
https://developer.android.com/identity/providers/calendar-pr...They get stored in a system table and you add accounts this way as well, right? This is also how DavX provides CalDAV support: it downloads the calendar and syncs it with the system tables.
In any case, at no point was I presented with an option to choose a default Calendar app or nowhere do I see a way to change that.