There's a throwaway line in LUMW about how the charge of the Chaos hordes tore up the coast of Norsca, and the drowning of the field by Manann is said to be the origin of the Sea of Chaos which is between Norsca and the Northern Wastes.
MF says more than 'Great Pyramid' - it says the defenders were driven up the pyramid to the "Diamond Throne". That seems like a clear description of Asuryan's Great Pyramid, the one from which He rules the heavens. This is also what OS appears to say but there it says "Flaming Phoenix, whom all had thought dead, returned from atop His Gleaming Pyramid", and I think it's possible to read that as Asuryan joining the battle and bringing the pyramid with Him, rather than the pyramid being the original location of the battle.
While the Shrine of Asuryan is definitely built in imitation of Asuryan's pyramid I don't think the Asur claim that it is the pyramid of Asuryan, as that pyramid is supposed to be in the Heavens with Asuryan Himself. Of course it could still be that the Shrine of Asuryan was built upon the place where the Gods gathered to fight Chaos or whatever, perhaps the place where the Qt Deific Interfaces were connected to or something like that. On the other hand our best guess for Qt is Quintex, which isn't in Ulthuan.