The issue isn't them being in a landfill, really - that has its own explosion risk issues, of course, but once it's in the landfill, I suspect glass would be worse anyways.
I haven't heard this claim about them exploding compactors, but rather of them exploding in incinerators (one of the many things one shouldn't burn). Irregardless, this isn't 'it'll start a trash fire' but rather, that they'd destroy a much more expensive piece of equipment and, in the case of the compactor, not actually wind up in the landfill they were meant to get to.