“the current mapping of '__builtins__' is copied into *globals* ”

That sounds like we insert each individual builtin name, i.e. globals.update(builtins_mapping). But my understanding is that it is the __builtins__ global variable that is affected:

globals["__builtins__"] = builtins_mapping

What about borrowing the wording from exec():

If the *globals* dictionary is present and lacks ‘__builtins__’, a reference to the current mapping of ‘__builtins__’ is inserted under that key . . .

See also Issue 26363, which also covers this problem.