Let's stack up Isolation and RE6, one is FP, the other is 3rd.
Alien Isolation you're shooting clones slowly walking toward you with a limited amount of ammo, click click oops you're dry so now you have to retreat, you're not invincible and you're at the mercy of the game to either have the enemies ease up off you or offer more resources. You're crafting items, tense the entire time and you're trying to survive through to the next area.
RE6, coming off a series that spawned the genre name in the first place you enter a room and 30 enemies swarm you, you don't need to escape or hide, you just strafe around shooting until everything on your screen is dead. Ammo management comes in the form of the player being bad because it's absolutely plentiful. Retreating isn't even an option because you feel like a god.
Even though Isolation is first person I'd say it's FAR more of a legit survival-horror game than something like RE6 or Revelations is. You have to have scares, item management and character vulnerability to belong in that camp. I'm no expert this is just how I've always looked at it, I use REmake as a baseline because that's the GOAT to me.
I hadn't played it, I've heard over the years it was a legit survival-horror game.