Thought the same. However, consider that it requires a single person of european descent mating with a single person of aboriginal descent to give to the offspring all european and aboriginal ancestry up to a few hundred years earlier. If the offspring mates again within the aboriginal population it starts spreading this ancestry. In a few hundred years it might have spread it to the whole population (even if not a single european gene is actually still present in the dna).

The point is: we consider ancestry a purely additive mechanism; in fact at each generation the amount of dna actually received from a specific ancestor halves, so that you can be of european, or asian or african descent and yet have practically no genes from that side.