From the original article:

  There were people announcing that our software will work 
  just fine on top of their technical stack, others posted 
  videos showing our software working somewhat on top of 
  an (IMHO idiotic) hack. It was decided on a mailinglist 
  I'm not subscribed to that I would walk people through 
  the KWin code base in a telco for adjusting KWin for 
  their technical stack. All of that without ever asking 
  whether we are interested at all. In case of the telco I 
  was not even asked whether I would want to participate 
  and whether I have time for that. I experienced this as 
  a constant pressure and a disrespect to our own 
  decisions.

It sounds an awful lot like Canonical expecting Gräßlin to spend his time on this project despite the fact that he's not interested in it at all.

  Isn't Mir supposed to have XMir as a compatibility layer 
  anyway? Wouldn't this mean that KDE should work on top of 
  it without extra porting work?

I believe the XMir compatibility is supposed to be at the application level, not the window manager/compositor level.

KWin is a window manager and compositor, which is fairly specific to the precise graphics stack underneath it.