You seem to have a misunderstanding of how continuum works in Windows. The interface is separate when you hook up a phone into a dock (it's a desktop interface) and the newer devices can also do it wirelessly.
Windows went with a "similar UI language" for all platforms but still maintained huge differences between each platform because that made sense. It was never the same UI.
As you said, unified interfaces aren't that important. I'd be perfectly happy with a phone that runs a Linux desktop while docked and gives me an Android GUI/Stack while in "portable mode". The only thing really needed then would be the possibility to open the Android part in a window in the "Desktop mode", so one could still do messaging etc. while docked.
Personally, I think that would already get us 80% there. The much bigger hurdle (and afaik the one Canonical had trouble with) is then establishing a market position.
a) Files - Dropbox b) Activity - Various browser sync for bookmarks, history (and passwords) c) Handoff - Mac is pretty good here. Windows has no equivalent.