So I actually talked about this in the calc, but we know how Astel did it and what he used to do it. Meteorite of Astel says its a manifestation of the power he used to level the Eternal Cities, while the description of Noklateo in Nightreign (which I go over as being applicable to the base game event) says that the destruction was done in a single night. Considering the fact that the meteorite attack can be held and is only limited by the Tarnished's limited FP, I'd imagine Astel just used this spell to destroy and sink them.
I think the biggest objection I have is that it foundationally assumes this was done in one sudden blast (such that we would scale a full value to an attack) when no dialogue about it suggests anything like that. Nothing supports the idea that Astel just nuked the earth below the cities and dropped them in free fall. If we were to assume it was an attack, as opposed to some form of magic or dislocation effect or whatever else, I don't think we'd have a good reason to take it as one attack.