Very well, you seem to definitely know this stuff better than me! I based my comment on this answer and getting this myself on Mint 21.3:
$ cat /etc/debian_version
bookworm/sid
But reading a bit closer, I think this is the key part:
That's how, for example, Ubuntu 20.04, released in April 2020, can be based on Debian 11 "Bullseye", which was released in August 2021.
So Ubuntu probably pulled Bookworm before it was released, and before it upgraded policykit. But it's still to some extent based on Bookworm. Does that sound right?