Hi,
I also noticed that my google calendar was not updating to reflect the updated abnb calendar. It's still not updated after about 12 hours.
It would appear to be a Google problem - see below
I decided to do a bit of investigation so I copied the URL link (obtained from ABNB export) and pasted it into my web browser as as URL (it is after all a URL). I then opened the .ics listing as a text file and bingo - you can read (with difficulty) all the date events that ABNB exports. Each event is bounded by BEGIN:VEVENT and END:VEVENT with a date end and date start.
There is also a userID a description and summary.
The Description contains the guest reservation code and the last 4 digits of their phone number.
If the event is a blocked off event there is no description.
The summary contains Reserved for a guest booking or (not available) for a blocked off event.
Here is an example for a self blocked off period from 19th-21st June 2020 (I have deleted the UID)
BEGIN:VEVENT
DTEND;VALUE=DATE:20200621
DTSTART;VALUE=DATE:20200619
UID:xxxxxxxxxxxx-xxxxxxxxxxxxxxxxxx@airbnb.com
SUMMARY:Airbnb (Not available)
END:VEVENT
Changing the ABNB dates and then refreshing the browser URL page immediately shows the date changes. This implies that any external app (Google calendar or otherwise) which interrogates the URL will see the updated dates. So the problem must be with Google Calendar not refreshing from the URL (even when hitting the refresh button inside Google calendar).
Sorry this doesn't really help but at least we can see where the problem lies.
The bottom line is that each organisation is only interested in itself and Google will not increase their refresh periods if it slows down their servers just to help out ABNB - it's not their core business!