Actually, looking into the implementation a bit, I can say there is definitely no bug here.  Line 153 basically works out to `y = x.__deepcopy__(memo)`, which is how the `__deepcopy__` method is documented[1] to be called.  Calling `copier(x)` instead would cause serious problems when a well-behaved `__deepcopy__` method then tries to call `deepcopy(self.some_attribute, memo)`.

As such, I'm closing this issue as "not a bug".

[1] https://docs.python.org/3/library/copy.html (see the last paragraph)