That's not true though, is what they're saying. Quake demo files are server-to-client packets, results of the simulation, not client-to-server packets, the inputs.
If you wanted to add random critical hits and random bullet spread based on the pixels in a live feed of a lava lamp cam, clients could still record .dem files and they would still work.
If you wanted to add random critical hits and random bullet spread based on the pixels in a live feed of a lava lamp cam, clients could still record .dem files and they would still work.