Melissus reaffirms Parmenides by explaining how there cannot be many without the existence of that which is empty, and since that which is empty is nothing, and nothing can't exist, thus there cannot be many.
It seems to me that they associate "nothing" (that which is not) with "empty" (not spatially extended) by definition, and then furthermore "nothing" cannot exist by definition. So, the only rationale for the nonexistence of that which is empty is an "intuitive" one rather than a logical one?