But then someone has to watch the recording which you can speed up by say 2-4x but not much more than that. This doesn't scale. The number of terrible candidates out there who have no business being anywhere near an IDE is insane.

I'd guess you do it like bot detection.

An online coding platform is going to be all js. You can track key press and mouse events and probably get a good idea of what's obviously real and what's not.