So, it'd be fun to be wrong, but given the word "fable" in previews and reviews and the eagerness with which reviewers are trying to avoid spoilers ... there's that.
But the trailer highlights a few simple, obvious things. The island has turtle-ish elements in its design that are visible in trailer excerpts. The red palette for the turtle also corresponds to the hair of the woman. So it wouldn't seem like a huge surprise if the island-is-the-turtle-is-the-woman-is-mother-earth or something like that.
After all, anime draws upon all sorts of traditions that include storylines like The Hakkenden (you don't really want to know, perhaps, if you don't already do). So castaway man has life with island-turtle-woman-spirit doesn't seem too far-fetched. It's super-squicky to a Western film reviewer, probably, who's unaccustomed to this kind of thing.
Again, it'd be fun to be wrong and not be able to call this just from seeing one trailer but ..