Lol I played that doom wad before and remember I got to that level and my jaw dropped. Wanted to make a post here but ended up forgetting about it. I played lots of doom maps that use music from other games before tho (one good one recently that used a track from american mcgee's alice) so its nothing out of the ordinary, Alexander Brandon composed some good music tracks that fit well into a lot of different environments.

(i'm also subscribed to that guy on youtube :P)