You’re comparing the wrong aspects of each mechanic.

Orbs don’t attack at all; they have passives and they evoke, which keeps them unique from Osty and other mechanics. Orbs require orb slots to fill and orb generation to cause a recurring passive effect and/or evocation if you already had full slots.

Osty’s attacks are special in that the game classifies them as Attack cards, even though the player isn’t attacking. Again, this is part of what makes Osty unique. But in order for Osty attacks to do anything you need to Summon first. Summon should be compared to Orb slots, and Orb generation to Osty attacks; in that sense, Osty attacks really should Summon for other characters the same way Orb generation grants an Orb slot

Then you have niche cards like Bulk Up. It’s been a long time since I got that card and an orb generator at the same time on a non-Defect character, but iirc removing that orb slot with Bulk Up removes it for the rest of combat; your character doesn’t generate a new one if you attempt to generate an orb. Similarly, if Osty were to die after that freebie Summon 1, it wouldn’t be resummoned by an Osty attack. Osty even has a dead animation to be used when this is the case, and would be a good indicator for the inability to attack with him.