The effect of temperature on the caffeine intensity
When coffee is being brewed in a moka pot, the water keeps evaporating until it reaches the coffee powder, then it condenses and falls down again, bringing some extracts from the coffee powder: it's just like a small reflux condenser.

When the pressure is high enough, the extract reaches the valve, and it escapes, giving the brewed coffee.

When temperature is too high, the pressure raises too quickly, and water fails to make a good extraction (just think about a reflux condenser which is kept running only for a few cycles).

If the temperature is appropriate, the water performs many evaporization/condensation cycles before it comes out of the valve, hence performing a better extraction (which, I guess, implies an higher caffeine level).

That's why usually a low temperature and a slow brewing are recommended.