So for astronomers, metallic means "behaving like everything else other than hydrogen and helium". In fact, I'm guessing "metal" (noun) came from use of "metal

So for astronomers, metallic means "behaving like everything else other than hydrogen and helium". In fact, I'm guessing "metal" (noun) came from use of "metallic" (adjective). The mention of helium betrays it - helium is what hydrogen behaves like normally "due to the insane pressures" (and heat), so what it basically says is just "not hydrogen".