In a different possibility that was foreseen, that Nethys had warned of, if things had gone according to plan, a Keltham acting with less time to prepare would have taken the shorter surer route: simply detonated an vast explosion above the Starstone Cathedral.
Keltham would (in that vision-of-possibility) have directly Wished antimatter into existence on that spot, in sufficient quantity that he could be reasonably sure that Aroden's Starstone-protecting fortress would be blasted down on the first try.
It would have been futile, then, to warn anybody but Teleporters to flee Absalom imminently. The coastlines of nearby countries would have received that warning.