I have a Motorola MotoG3 running Android 6.0.1.
I was not quite aware, that this should not be possible, so i've set up a folder to sync on my phone. I did it from the web interface (I've changed the bind address in the app, and logged in from my pc) - I set up the folder first on my PC, then shared it to the phone and confirmed through the web gui of the phone syncthing instance, giving the path to the folder (/storage/"card"/Music).
Aaand... It works, flawlessly. It detects changes on the card, it can write new files, modify them - all is well.
So why is it said, that it doesn't work?