It looks like what those older games felt like.

If the author realizes it, he could make the next Valheim hit.

No polygons in Doom/Wolf3D.

The example from Back To Saturn X2 can be played on a software rendered engine with no concept of polygons at all.

Isn’t it referred to as 2.5D ?

More like a bunch of cardboard figures raised extruded and raised up to look 3D. You couldn't stack floors in Doom for instance.

2.5D would be the semi top down games such as most beatem-ups allowing you to roam around instead of just going left/right as the typical platform or action game, or most SNES RPG's.