Not sure who you are asking, but in my engine, compression was always fine. That particualr engine did appear to be detonating...I could hear it when it came down to idle like a sizzling sound. That would occur at about 325degF.

Engine parts won't melt at 400degF, but the mixture could be so lean there isn't enough lubrication, or the combustion chamber gets so hot, you get pre-ignitiion or detonation, not sure which.

It would appear the occasional slip that allows high temp running doesn't necessarily ruin anything. Depends on other conditions, like the % oil in the fuel. Even hot, it has to be rich enough to keep running.

I'm unsure of the exact numbers now, but I know with another engine, I would always be running around 280degF on grass and was surprised that with the same tune, same day, etc., I could only get it to 225degF on pavement. So it can get hot, but still not be running lean.