> If exec gets two separate objects as globals and locals, 
> the code will be executed as if it were embedded in a 
> class definition.

That's a misleading comparison because a class definition intentionally supports nonlocal closures, which exec() doesn't support and shouldn't support. For example:

    a = 1

    def f():
        a = 2
        class C:
            print(a)

    def g():
        a = 2
        class C:
            nonlocal a
            a = 3
        print(a)

    >>> f()
    2
    >>> g()
    3

exec() executes as module code. Using separate globals and locals mappings doesn't magically change how the code is compiled and executed to make it equivalent to a class definition. To understand the case of separate globals and locals, just remember that assigning to a variable by default makes it a local variable, unless it's declared as a global. Also, class and function definitions are implicitly an assignment, which by default will be local.