I can see the summoning thing being read either way, and honestly, with how ludicrous the power of summoning is, requiring other models of the same type to already be on the table is, if anything, an insufficient check on that power.
Remember, dispelling can only even be attempted if the wizard that wishes to try it is within 18" when the spell is cast. And we have 3 available stacking bonuses to that casting roll that any of our wizards can benefit from - corpse cart's lodestone, Morghasts, and Mortis Engine's blasphemous tome. Taken together, even common necromancers auto-pass the DC 5 needed to cast most basic summoning spells. A backfield coven of 3 necromancers with the listed support units puts out 30 to 60 skeletons, zombies, or ghouls a turn, with very little the opponent can do about it.