Yep, I thought that was the case by looking at the screenshots (they're too crisp to be ray-casting)
They're too crisp because of the high resolution (most old games ran at lower resolutions), a good raycaster would look like that too. I have played Doom engine based games before at 640×480 and yes, it does look as crisp as that (albeit some bugs appear with huge sectors due to limited fixed point precision... much bigger than anything here though).