That's true; with a cap of 250 Mbps for the DCP stream, and a 4K frame size of 4096 × 2160, that's ~28 bits per pixel (if the soundtrack is disregarded). There might be a small amount of subsampling or run length encoding going on, but it's entirely plausible to distribute and play an uncompressed film on a professional projector.

EDIT: Forgot to factor frame rate into the math. Divide that by 24 frames per second. That got me curious, so I looked into it and found they're using JPEG 2000 on each frame with no inter-frame deltas. Essentially like a constant stream of I-frames.