That's not true at all. Wolfenstein 3D was a raycasting engine based on cubes. Doom used BSP trees and allowed for much more varied level geometry without right angles everywhere (although there still were no sloped floors).
Doom added semi-dynamic lighting, arbitrary line angles (wolf3d was only grid-based), and moving segments.
Maybe not as large a leap as from Doom to Quake, but at the time it was significant.
Doom-to-Quake was a bigger technological leap, going from a hacky 2.5D raycasting engine to a fully polygonal 3D environment and renderer.
But Wolf3D-to-Doom was a bigger gameplay leap. Doom could do enormously more with level design and the visual experience than Wolf3D could. Quake being fully 3D didn't really do all that much for the player experience over and above Doom.
But the engine was highly mod friendly. I think i have at least a couple of CDRs somewhere (if they are still readable) Stuffed with zips downloaded from Fileplanet.
Everything from grand projects like Team Fortress, to small one man mods that was not much more than a collage of weapons.
This in large part because the game logic was implemented using quake-c, and the compiler was freely distributed.