So looking at Domey's statement and watching the press conference, the Nix's only issue is that AFC jumped the gun by announcing AP's signing while it is currently not possible, and the proposed changes to the rules need to be ratified before it is. And to be fair to the Nix, this is the only reasonable complaint.
It does come across rather arrogant for AFC to have done this, either that or they are trying to pressure Football Australia into deciding it in their favour by making it a horse has already bolted situation.
Regardless, I wouldn't be happy if my club was doing things in this manner. Could they instead have announced the prospective signing subject to the necessary official processes, saying that they were hopeful of them being ratified, and said that AP would be training and playing friendlies in the meantime? This would also be far less embarrassing for them should FA reject the proposed changes.