My first thought agreed with yours, Bill; then I thought about how small amounts of heavy elements might reduce luminosity.
Luminosity might be reduced by something less transparent covering the outer surface, and this, as you point out, would tend to increase the temperature and therefore the rate of fuel consumption. It would also require large amounts of whatever it was, which is not what we are dealing with.
We are dealing with small amounts of heavy elements, which one would expect to find in the interior of the star. This might reduce the rate of reaction, thus resulting in lower fuel consumption, and reduced luminosity. So it is the lower fuel consumption that causes the reduced luminosity, rather than the other way round.
Perhaps “traps radiation” was not the best term to use. “Inhibits radiation” might have been better.
Just a thought while we wait for the experts..
