Correct, but Alaska is attached to the landmass of the American Continent. So according to most poskim follows the rest of the continent.
It is similar o the situation in Australia. Melbourne for example, is east of the halachic dateline, but Shabbos is observed on Saturday like the western parts of Australia, because you can’t have a situation where a land mass is separated by a line with regard to the onset or end of Shabbos. Tasmania is an island and separate to the rest of the continent, and therefore, would have Shabbos on Sunday.
Interestingly, Rav Moishe Sternbuch holds, that in the ocean off Melbourne Shabbos is on Sunday, and therefore one may not swim in the sea on Sunday. Rav Elyashiv holds that as far as you can stand in the water is still called land.