ummmmmm have you been to the floating land in celestia? its surrounded by water. so its not really on celestia. and theres no water under skull island. theres a reason they call it the skyway.well thats all.
Sorry, but Skull Island is its own separate world, when sailing between worlds on the spiral threads I have seen Celestia in the distance. I think you can see Celestia when sailing from Skull Island to Mooshu? However, Skull Island is very similar to Celestia and possibly Skull Island is situated close to Celestia in the Spiral causing both worlds to have similar environments. Another possible reason is that before the Spiral was formed when all the worlds were still joined together as the first world, Skull Island and Celestia would have then been connected.
skull island is on Azteca, I mean I, I don't really see how you missed that
not necessarily. there were azteca RUINS but not actual azteca citizens in skull island. mummys were there but you should count that there were living breathing aztecas until the shabalba. so its most likely the aztecas moved out of skull island for some reason.