Derren said:
In a magical setting this can be disputed. It still doesn't change that the Archons are made out of living fire.
But they are not
flames. They are solid, can be touched without suffering burns (although it seems they're uncomfortably hot), and so on. They are creatures of elemental fire, not fire itself (which is, after all, non-living and non-sentient).
Also I really doubt that all molecules are harmed by energy extremes. Objects, made out of molecules maybe (which I don't think is universally true) but molecules? Ice might crack when it gets too cold but most of the ice molecules are still intact.
Sufficient energy will first break up the molecules, then ionize the atoms (turning them into plasma), then disband the atomic nuclei, until finally, at Big-Bang temperatures, even the protons and neutrons fly apart into quarks.
None of which is really relevant for the discussion. You could very well argue that matter in the D&D universe is contiguous, and can be divided into infinitely small portions, or presume that there are elemental atoms, or whatever.
In any case, it's futile to try and argue from real-world physics as to whether ice archons should be able to withstand infinite cold, or fire archons take any amount of heat. The game designers have decided that absolute immunities are either rare or non-existent, because it they believe it makes for a better game, so obviously sufficient extremes of energy are harmful to the archons.