And even if that was true and Alaska joined the United States via democratic mandate, all the voters would now be dead.
Why should the democratic mandate of a bunch of dead people be held more valid then living people?
Besides, realistically? In some alternate unvierse where A. Alaskans want independence as a majority and B. Alaskans achieve independence, I'm sure American oil companies would just find a way to control all of Alaska's oil. Who needs invasion when you can just buy the political class?