Suppose an Abyssal sorcerer used shades to duplicate summon monster VIII. Could they conjure 2 Hezrou?
I would vote no. You are not using a summon monster spell. You are casting shades. That particular casting has a similar result as casting summon monster, but it's not actually summon monster.
Added Summonings (Su): At 15th level, whenever you summon a creature with the demon subtype or the fiendish template using a summon monster spell, you summon one additional creature of the same kind.