It is a no brainer. The observation that exhaust valve temp - as a function of mixture - begins to fall slightly rich of peak EGT does not in any way invalidate the theory that valve temperature correlates with some combination of CHT and EGT. In fact, as the working fluid is the heat source and the cylinder head is the heat sink, it cannot be otherwise.
The increase in CO2 as a percentage of the atmosphere
correlates with the decrease in the the number of pirates (as any practising Pastafarian knows). But the decrease in the number of pirates has not
caused the increase in CO2 as a percentage of the atmosphere (as any practising scientist knows).
What
causes the CHT to go DOWN even though the EGT keeps going UP?