Man, it's almost as if you know nothing about what causes mushroom clouds to form!
I'm having trouble determining what exactly you're trying to get across here. Was it because I didn't add "large" infront of "mushroom cloud"? dang, was really hoping the context would help there..
Very few explosions have been as large as nuclear explosions. Those are megaton explosives. Again, he said a large explosion. This is a chemical plant, I'm sure it has the yield to do this.
no, he's saying it's unreasonable to assume or at least consider the possibility of it being from a nuclear explosion. it's not unreasonable to have it at least cross the mind- the shockwave, considerable size and light produced, etc. of course I acknowledge that other things besides nukes can create large mushroom clouds- MOABs, this chemical plant, anything that can generate lots of convection. but lots of people will connect "big mushroom cloud" with nukes, because that's a familiar characteristic of them.