If you can class this story at all, it's simply "Fantasy" , the overarching term derived from the early 19thC "Phantastical Tale".
Yes, yes, nowadays the whole field is "classified" into so many subgenres that it makes Metal afficionados with OCD twitch in envy, but it's all one type.
Whether it's horror, sword&sorcery, space opera , hard SF, or any mixture, it's all Fantasy. Something that's very clear when you read the old masters, as they were only limited in putting in by what the editors/publishers desired to see.
Hell, Asimov started his career by adapting ancient Greek fables with spaceships and pewpew. Some of his most famous "Early Work" are stories over 3000 years old that he translated while in College from greek into english, with a twist, because he was bored and figured out he could make a couple of bucks pitching them to those newfangled "Fantasy" rags.
As far as this story goes it's hard to say, because like any good story it defies any attempts at stuffing it in a labelled box...
If at all, I'd class it as Space Opera with a "Uninhabited Island" Survival Story main plotline. With the mandatory local Savages/noble Savages.
Which, in terms of "modern" literature, would mean this one is the greatgreatgrandchild of Defoe's Robinson Crusoe.