When I put the Motopress export calendar link in my browser – an ICS file is downloaded. I can import that splendidly to Apple calendar, AirBnB, and VRBO.
When I subscribe to the calendar just using the SAME link, I can only see SOME of the appointments in Apple calendar, VRBO, and AirBnB.
Both bookings in provided .ics file have already passed, this may be the reason why they are not displayed in external calendars after synchronization. Try adding some test bookings for future dates and check whether they appear in external calendars after syncing with them.