The pigeonhole principle doesn’t say what you want it to. It guarantees that some ancestors will show up multiple times on everyone’s trees; it does not guarantee or even suggest that every ancestor present on anyone’s tree is present on everyone’s trees.
That aside, it’s not clear that the descendants of Salazar Slytherin would mix sufficiently with the rest of magical Britain in 1000 years of wizard generations (possibly longer than Muggle generations, given differing lifespans) for the paper’s findings to apply. Running with general experimental assumptions is not effective for specific and extraordinary cases.
The pigeonhole principle doesn’t say what you want it to. It guarantees that some ancestors will show up multiple times on everyone’s trees; it does not guarantee or even suggest that every ancestor present on anyone’s tree is present on everyone’s trees.
That aside, it’s not clear that the descendants of Salazar Slytherin would mix sufficiently with the rest of magical Britain in 1000 years of wizard generations (possibly longer than Muggle generations, given differing lifespans) for the paper’s findings to apply. Running with general experimental assumptions is not effective for specific and extraordinary cases.