I always assumed it was intended effect, otherwise they’d be too overpowered.

Judging by sorear's list (if comprehensive), monster summoning is not included in monster spellcraft, so it could be an oversight or it could be intentional. Seems odd that it wouldn't count for dampening purposes.

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".

Right, that would make sense.