The wikipedia article mentions that there are similar stone tools above and below the ash layer in India so that it may be that the Toba eruption did not kill everyone in India even though it was local to India.

So it could be could be coincidence on the timing of the Toba eruption and the genetic narrowing down of the human race at almost the same time. But something happened 70,000 years ago, if not Toba, what was it? Maybe the onset of the ice age + Toba + other things occurring close together in time.