Ordinarily an impact on the size of either should arrive once every hundred million years or so. That two would independently arrive 300,000 years apart is an extremely unlikely coincidence.

The Nemesis theory says that every 26 million years Nemesis arrives and knocks a bunch of stuff out of the Oort cloud. Most of it is swept up by Jupiter, but the Earth goes through a period where large asteroid impacts are likely. It is a much smaller coincidence to have 2 large impacts in a period where large asteroid impacts should be common.

Therefore this result is more substantially likely under the Nemesis hypothesis than it would be without the Nemesis hypothesis. Which implies that the conditional probability of Nemesis given this observation is higher than it would be without this observation. Which is a fancy way of saying that the observation is evidence for the Nemesis hypothesis.