It's labeled as "action, adventure, RPG" on Steam, but I've never seen anything from the devs claiming it's an RPG. It's not just combat though, there is puzzle solving as well.
I think the non-combat bits probably lean more towards immersive sim than RPG. However, it's a first-person shooter first and foremost just like Bioshock was. The difference is that a lot of the combat here seems to be melee, or a combination of melee and ranged, as opposed to mostly ranged.
As for length, the devs said around 25 hours standard, 35-40 hours for a completionist run.