I read the other issue, thanks.

What do you mean by "portable version of mountpoint"? Is there a portable version of mountpoint or was it more a figure of speech?

Concerning the issue itself:

What are the priorities with the behaviour of ismount?
Is it more important to be consistent with the system it is run on or is it more important that every python instance on every system would return the same values?

I see merits for both, I'm just curious what the python developers value more.

In case it is the later I propose to change the docs on ismount again to include that it disagrees with Linux on nested btrfs subvolumes.

If you like I can make a pull request for that.
My favourite solution would be to have agreement with linux on this but at the moment I have no idea how that could be implemented without resorting to Linux native utilities.