[Dovecot] Question about "target is inferior" type of message

We're converting UW-IMAP users here to dovecot. Some of them have seen
the error message issued when Thunderbird clients try to delete folders and I understand
what's going on (from previous postings) that Dovecot doesn't support mbox subfolders within
folders.

What puzzles me is that many of these users already have folders
within folders (I've found typically 2 levels of nested folders within their main Mail
directory). And they're not complaining that they can't read the messages in those folders. If UW-IMAP
supposedly never supported this for mbox format, and Dovecot doesn't support it, how is this
possible under Dovecot? (We don't have any maildir format mailboxes here at all).