I don’t believe that’s true. DCSS is mostly true procedural generation from a seed against constraints. There however some special fixed layouts for final floors, and small predefined vault rooms.
I've played an absolute fuckton of DCSS and it is true. There are small "blocks" that you'll see over and over that are exactly the same. The best example I can give is the room Crazy Yiuf hangs out in with the tree hallway leading to it.