If we look at this issue from an angle "ethics is memetic system evolved by cultural group selection", then I guess it makes sense that (1) systems promoting helping your cultural group would have an advantage over systems promoting helping everyone to the same degree, and (2) systems that allow to achieve the "ethical enough" state reasonably fast would have an advantage over systems where no one can realistically become "ethical enough".

The problem appears when someone tries to do an extrapolation of that concept.

I am not sure how to answer the question "should we extrapolate our ethical concepts?". Because "should" itself is within the domain of ethics, and the question is precisely about whether that "should" should also be extrapolated.