https://lists.freedesktop.org/archives/dri-devel/2015-Octobe...
Back then the probed panel resolution (EDID retrieved by radeon) was 3840x2160, which doesn't make any sense because the panel actually has 5120x2880. It's unclear if this was because of missing/broken MST support in radeon (Multistream Transport) or if Apple used some proprietary mechanism to squeeze the panel resolution in a single stream transport. You may want to contact the person who started that thread (Andreas Tunek) and ask if the situation has improved with contemporary kernels.
It's the same reason why, for example, Windows sees it as a 3840x2160 panel as well.
I guess I'll wait for Apple to actually announce new desktops before I seriously look into the driver situation...
[1] http://forums.macrumors.com/threads/bootcamp-update-5k-windo...