You can see this in Ruby where methods don't close over the environment, but blocks do, clearly Ruby methods aren't closures, not even when reïfied into a Method, whereas blocks are closures.
It's like code expecting rectangles wouldn't mind to get squares, but a code expecting squares will definitely do mind getting rectangles.
It is actually not that obvious since in OOP neither a square or a rectangle type is properly a subtype of the other. [0]
I understand how implementation-wise it could be convenient to treat a function as a closure. Although, I would prefer some other type, eg:
functoid := closure | function
You might define the interface of a closure as `closure(f: Function, env: Mapping[str, Any])`, or in other words a closure is a function and a mapping of variable names to values. That mapping can be empty, but that's different than the different interface `not_a_closure(f: Function)`, which is incapable of closing over anything, including the empty set.