Intel's CPUs are likely not modified (very much). All Intel CPUs automatically downclock themselves when they start to overheat as a self-protection measure.

It is up to the case-manufacturer to provide adequate fans and cooling for the chip. Otherwise, Intel's chips are forced to protect themselves and start to slow themself down.

This seems the same as someone selling you a 4K monitor (for 50% more than other 4K monitors) without ever mentioning that you can only actually have 4K for 5 min max, otherwise it's QHD. It sure sounds like something that should be illegal to me...