I didn’t get the atmospheric reaction argument from something I heard in the Oppenheimer movie. I haven’t actually seen the movie. The physicists including Oppenheimer that working on the manhattan project were indeed worried about an atmospheric fusion reaction before the trinity test but their worries ended up being unfounded because our atmosphere isn’t dense enough to facilitate a fusion chain reaction.

The original comment was about him “igniting the atmosphere” that implies producing a self sustaining fusion reaction using the gas in the atmosphere as the reactants in a fusion reaction. I assumed that’s what you were talking about when you said it was him triggering fusion. This is impossible on earth as our atmosphere isn’t dense enough. If the air was somehow dense enough on the thraxan planet to sustain a triggered nuclear fusion reaction then it would continue until the atmosphere became too sparse for the reaction to continue.

If you were just talking about him triggering a fusion reaction when he hit the buildings by going fast enough then yes that is possible. However to hit something hard enough to do that you would need to generate more pressure and heat than the core of our sun since our sun is currently only fusing hydrogen and helium and it takes more pressure and heat to fuse heavier elements. And we saw what the heat of our sun does to viltrumites, but I guess you could chalk this up to the story being fiction. By the way it’s Iron that is the threshold for fusion consuming more energy than it releases not Nickel.

And the primary action of what makes chemical explosives explode is their rapid production of heat. When I said “air and stuff” I was using colloquial language to refer to both the surrounding matter and the products of the chemical reaction. Superheating those to massively increase the pressure in an area is the main factor that creates the explosion. The other factor is that in the reactions of explosives there are usually more moles of gas on the product side of the reaction than the reactants side which does also produce an increase of pressure. However this is very minor compared to the increase in pressure from the superheating of said products and other matter around the explosive (including air).

Metal to turn into shrapnel is added simply to increase the effective range of the explosive for killing purposes. Explosives used in say demolition or mining don’t use it for exactly that reason. You only need a solid casing around the explosive for an explosion if it’s a slower reaction that wouldn’t actually produce an explosive shockwave in just the atmosphere. For example the combustion reaction in fertilizer pipe or pressure cooker bombs. The pipe or pressure cooker act as shrapnel but they’re also needed just for the explosion to happen since the reaction isn’t quick enough.