In Python 3.9 the binding is more late-ish binding, than true late binding.

Because globals['__builtins__'] is cached for each function activation, executing functions don't see updates.

Example:

>>> def f():
...     print(len("test"))
...     bltns = f.__globals__["__builtins__"]
...     if hasattr(bltns, "__dict__"):
...         bltns = bltns.__dict__
...     new = bltns.copy()
...     new["len"] = lambda x : 7
...     f.__globals__["__builtins__"] = new
...     print(len("test"))
... 
>>> 
>>> f()
4
4
>>> f()
7
7

True late binding would print:

>>> f()
4
7
>>> f()
7
7