Verified answer

Well,

Gravel, if its is made from rock, is not combustable since its mainly oxides that are already (burnt). Could be what he wants is a surface that is not permiable, may be afraid that oil and gas will go into the gravel and go into the water table or accumulate in the gavel and catch fire. Poured concrete is condidered impervious for these things and it won't let things soak through. Maybe that's what he means

everything burns if you heat it up enough.

probably not within what most people have in mind, though. gravel dust can be explosive, like grain dust. probably not the problem either.

gravel will soak up oils and fuels dripped on to it, making it flammable over time. that's what the inspector means.

no