The way the underlying engine works is what's significant, not that it's a dynamic game design element. Descent levels are just a graph of cubes where a portal rendering algorithm runs per cube instance in a flood fill that spreads from the camera origin until there's no more unclipped portals visible on the screen. You could totally make a portal gun mechanic in that engine, they just didn't want to. Well beyond the hatches that open and close, which do case an end to the portal rendering cell walk when closed.

There's nothing particularly novel about Portal (the game) style portals in rendering. Racing simulators did the exact same thing for rear view mirrors for decades prior. It's just another transform in the scene graph clipped to a subscreen area.