I dunno, I think it's pretty hard to keep cheating as a developer. How would a developer even cheat to get promoted? I mean, a PM I can see survive for longer, but a SDE?

I actually had this issue at my current job where we hired someone who seemed great and who did really well on his interview exercises but then once he started working... well, it was pretty obvious he did not know what he was doing at all. So he was let go straight away. There was no way he could cheat his way to stick it out for longer than a month.

It's also possible they can understand solutions provided to them and even implement but not be able to come up with solutions under strict time pressure.