Their basic premise is wrong, if bias continues to exist after the selection event in question.
For example, if YC had (hypothetically) a real bias against black or women entrepreneurs, it is almost certain that future funding rounds, as well as all possible exit scenarios, would exhibit very much of the same bias.
In which case, the future "performance" of those candidates would be poor, and by PG's definition unbiased even though the only meaningful result is that YC is no more biased than subsequent performance evaluations.