On the face of it, he has a point. ACNA is not on the list, not named in the Communion's book of Anglican life. Foley Beach should not figure in this meeting. End, of, story.
But what Mark's post does not appear to understand is a view widely held across other parts of the Communion - so I understand - that TEC has left the Communion (cf. reiterated talk in past years re 'walking apart'). It is Ecclesia Non Grata. How then is the ABC to proceed to call a meaningful meeting of primates if his list of Gratae Ecclesiae is different to the list held by others (at least as written in their own minds)? There is no Communion if primates do not turn up en masse. If inviting Foley Beach is the way forward to some kind of meaningful primates meeting or Primates Meeting then that is, to coin another term of diplomacy, the realpolitik of the situation.
I vote for Justin's realpolitik.
Forget the list. Think about relationships!