Heat, the by product of microwaves, will kill some bacteria and break down some toxins. So in some narrow range of conditions, microwaving a slice of pizza will decrease the chance of food poisoning. In a much broader range of conditions microwaving won’t help. But probably the conditions for a slice of pizza sitting on a table for a while are already within the safe range to start with.
That also assumes that pizza is not a magic food which never goes bad.