Still new to lightning, but after some more reading ...
Yes, it is not possible and the watchtower may only close the channel in case of breach.
By having the same seed on both mobile and desktop the channel appeared automagicaly so it can be force closed in case the mobile is lost.
My expectation was some sort of a mix between watchtower and a personal server running 24/7 on the desktop and the mobile client using the LN channels with a watching-only wallet (or different seed), but it seems this is not (yet) possible