Monster summoning isn't treated as a spell for other mechanics as well. Eating blink dog corpses never increases your mana or PP, which is a possible corpse effect for spellcasting monsters. The summoning ability of monsters with corrupting melee attacks can be removed with a potion of cure corruption, while spellcasting abilities are unaffected. There's a limit to how much summoning can be used, which doesn't exist for monster spells. And of course, monster memory lists it as a separate ability from spellcasting: "able to summon help" vs "able to cast spells".