That is pretty easy to resolve. Eve certainly is the mother of all humanity in the GAE if we use a textual definition of human: AE and their descendants.
Moreover, there are three important linguistic points(@AllenWitmerMiller):
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Whether or not there are people outside the garden, there are contextual bounds to the title. At that point in story, Eve is not mother of Adam and she is not mother of all the animals, even though they are all living.
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Whether or not there are people outside the Garden, the title must be being used anachronistically, because she is not mother of anyone when it is given to her. Therefore it must be understood as: Eve would become mother of all the living.
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Of course, Eve does become mother of all people across the globe.
Together, it becomes clear that this title can not possibly construed to rule out people outside the garden. It might be a good argument for universal descent from Eve at the time Genesis is written down, which would push more ancient the latest date allowable for Adam, but that seems somewhat of a concordist reasoning.
Regardless, if AE lived at 10 kya, then by 3 kya or so when Genesis is written, by then Eve actually is mother of all the living any ways.
So I can’t see how this is a remotely plausible objection.