I would say they didn’t need to demo anything, because if you are gonna use the output code live on a demo it may make compile errors and then look stupid trying to fix it live
If it was a safe bet problem, then they should have said that. To me it looks like they faked excitement for something not exciting which lowers credibility of the whole presentation.
We know exactly why, it is because floating point operations aren't associative but the GPU scheduler assumes they are, and the scheduler isn't deterministic. Running the model strictly hurts performance so they don't do that.