The point is that there are several countries that are not EU members that are in EASA. Whether the UK wishes or would be allowed to use the mechanism they employ does not alter the fact that the statement "is open only to EU members" is false.
Both sides in the negotiations have stated that their preferred outcome is for the UK to remain in EASA and several other bodies, and negotiations on this are ongoing. Yet this paper dismisses that in a footnote and concentrates entirely on the "no deal" option. So the conclusion I draw is that publishing this now is either showing the world that EASA have considered all eventualities - "bureaucratic arse covering" - or deliberately trying to give the impression that no deal is possible - "political **** stirring". It would appear from this thread that if that was the intention it has been successful.