Paul McKeksdown,
You can't look at it like a constant throttle or cooling air issue, because that is not how the engine actually is used. The author of that piece really does not understand how an engine operates, or at least does not understand how to tell someone how it operates! To explain what is happening, you must consider the need for a fixed amount of power, and what the engine must do to the air it eats and the fuel it burns to make that power. When the pilot needs a certain amount of power, the engine bends over backwards to deliver it, varying throttle and gas generator speeds as needed to make the power. The engine is only an expensive device that heats air and then extracts the heat from it, that is all. The reason the engine runs to higher temperatuures when it eats hot air is NOT that the throttle is more or less open, it is because the air mass times the difference in temperature makes the power, so a hotter package needs higher temp to make the power, as does a lower mass need even more difference in temp to make the power. Since hotter air has less mass per unit volume the given volume must be heated up more to make the power.
If you run it at lower altitude, the air is thicker, so more mass means less temperature increase needed, and it runs cooler.
The engine is affected by density altitude, but not like a wing or rotor, because the density is only part of the engine's equation. The temperature itself is also a problem for the engine. A wing or rotor does not care about temp, it cares about density, so it behaves relatively constant with respect to constant density altitude. An engine does NOT behave constantly regarding constant density altitude, because the absolute temperature of the air package is important to the engine, regardless of its density.