From a Duplicati code point of view, processing looks to me to be more sophisticated than LucidLink said although what’s now an IsClientPosix test used to be misleadingly named IsClientLinux, yet true for OSX. Possibly the name threw them off, or maybe they just didn’t read code right, or even maybe I’m not either.
Quota() calls GetDrive() which calls mono GetDrives(). It looks through the results to see if it can find the drive containing your path. The first problem is that GetDrives() on mono misses lots of drives. That was my first idea, although I didn’t see why / would give all zero values (and df shows that it doesn’t). I wasn’t actually expecting mono to find LucidLink and then fail to get space, but per your test, that’s what it does.
The short drive list is possibly hinted at by Xamarin Bug 11923 - System.IO.DriveInfo.GetDrives() returns a single null drive, and current version of add_drive_string (6 years later) might be here and still seems to let very little in. I’d have to look at the output of the mount command or cat /proc/mounts to see how LucidLink managed to escape the heavy filtering. Oddly I couldn’t find any complaints filed in mono issues about that, and one could debate what’s a “drive”, but to me lots of the layered UNIX/Linux/OSX mounts should qualify.