If IQ tests are 'culturally biased', then we would expect the highest scoring group to share the same culture as the test writers.

This assumes that if a test is culture biased, it must be biased in favor of the culture as a whole. A test can be culture biased by hyper-valuing a set of skills prominent in one culture, even if that skill set is stronger in some other culture. If IQ is biased, say, toward "academic culture," even though this is a feature of "white U.S. culture" it may be even more a part of East Asian culture.

What I think your argument shows is that the tests aren't intentionally biased in favor of one culture specifically. In fact, the studies of early IQ testing shows there was intentional bias (not so much today), but rather than being in favor of the dominant culture, it was against the cultures of particular immigrants. (I'm speaking of the Army Alpha tests.)