I've not seen all of the series but from what I have seen nothing seems particularly fantastical or science-fiction-y, the premise is that somehow the Axis won the war and have dominated the world.
I don't remember anything that would in my mind constitute science-fiction, no advanced technology or weaponry, seemingly most things are less advanced if anything.
To me this seems like it's more fiction than science-fiction.
https://scifi.stackexchange.com/questions/tagged/man-in-the-high-castle