Yep, that is accurate. There is also a command/API route to find the path on disk iirc.

In my setups it just was easier to use fixed paths (or relative to project dir) from a permissions management perspective. Backup tools did not always have to/should run as root which is helpful on machines providing multiple distinct services.

Putting borg or a similar tool in a container that is part of the compose manifest file can also help. I haven't seen this used in practice though yet.