This statement is not strictly true but in context correct. Any volcanic eruption is full of really nasty gasses like carbon monoxide, sulfur, chlorine, and eve

This statement is not strictly true but in context correct. Any volcanic eruption is full of really nasty gasses like carbon monoxide, sulfur, chlorine, and even fluorine, to name just a few. This comes from the content of the earth already (well, all the garbage comes from the earth originally also). But like you implied, adding the tiny amount of garbage generated by humans to the mix to an entire crustal column of rock would have insignificant impact on what comes out on top.