If you have a system that can function but it's missing opposable thumbs, than all tools requiring grip and use of the opposable thumb are not "overhang" for that system.
But for general intelligence, absolutely everything is overhang or quickly becomes overhang across all layers of organization.
So for a limited system, overhang exists only inside its current action space. For general intelligence, the action space itself is editable. If a capability is missing, the system can build, borrow, automate, delegate, or redesign around it.
Opposable thumb needed? Opposable thumb built.