I'm pretty sure that Jagex has always been consistent in their policy that the game takes precedence, so if they say anything in outside sources such as the books or a Q&A, anything the game does that contradicts that becomes canon. Their statement that Elder Gods cannot be killed has
always
been subject to change, and has never been backed up by any source of canon.
Plus, it's very heavily implied that Wen was directly responsible for corrupting Mah's core, so it could be that the only way to make it possible to kill an Elder God is through another Elder God's power.
22-Nov-2016 16:35:21