The reason I think it is unreasonable is that:
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There is a known history of contact in recorded history.
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The reasons for their isolation do not extend that far back in history, and human culture is not stable over millinea. An isolated tribe would need to be totally isolated trough changing external factors, generation after generation, somehow overcoming a pervasive human trait of socializing and exploring. It defies my imagination to envision this with out dramatically altering our understanding of human nature.
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Every time we have look at whole genomes of supposedly isolated groups, every time, we find the isolation was mythology, disproven with direct evidence.
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Short of science fiction scenarios, it is hard to envision strong evidence against mixing in the distant past.
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Nearly every ancient DNA study shows heretofore unknown mixing in the past, often over longer distances than imagined.
If a scenario like this arose (A Science Fiction Riddle) many theological questions would arise. It is something at this level that would raise questions. Even then, i would fall back on CS lewis’s Religion and Rocketry, put forward the best ID argument ever, and note that Scripture says “to the ends of the Earth, not the ends of the galaxy.”