See that's the thing, I judged it by its cover, it looked like an isekai to me and I kept far away from it. This is some serious hype, gonna check it out.
IGN calls Frieren an Isekai anime on their articles and they got cooked by the comments for that.

The difference between Isekai and fantasy is that Isekai usually has the main character transported from a modern world to a medieval-like setting world of magic and mythology and stuff like that or straight up die from getting hit by a truck or shit like that and gets reincarnated into a different body in a medieval world and grow up there.
Fantasy genre is just medieval setting like off the bat, no modern world influence or anything like that until further explained.
But yea man, I hope you'll enjoy it.