I just had a look at the code.
You are right, purge will trash everything in the directory regardless of whether it is trashed or not. It says to google "trash this directory" in a single call.
So using --drive-trashed-only with Purge is likely not to work as the user expects! What I can do is make Purge exit with an error if it is called with --drive-trashed-only. Can you please make a new issue on github about this and I'll make the fix - thanks!