Hi, glad you enjoyed the game.
The game is made entirely in Unity. Sprites use a billboarding technique for a 3D effect (and some shader tricks to work with the geometry properly). The 3D maps are made with homegrown in-Unity editor originally design for tactics games. If you're interested, the source for all this is available at https://github.com/psywombats/scream-2021 .
Neat, yeah 2.5 isn't much more difficult to pull off than 3D, provided you've worked with 2D stuff like spritesheets.
And yes the game was made in about two weeks. But it's a team of three, I work professionally in Unity, and I cribbed a bunch of old code to get some basic systems (mapping, cutscenes, sprites, audio) all going. ...and didn't get much sleep haha