I agree but we can make an educated guess based on what we know of life in Pompeii.
Just to make an example, I come from a small-medium sized town in Lombardy. On Saturdays about 33% of those gathering in its center are clearly recent Middle Eastern and African immigrants or children of Middel Eastern or African immigrants. Mind I'm not including less apparent immigrants like Eastern Europeans.
What would happen if a sudden cataclysm occured on those days, causing the death of all the people going for a walk? Some scientist exhuming 10 or even 20 random samples' remains and analysing their DNA 2,000 years later could come up with some VERY wild interpretations about the genetic make-up and the level of cosmopolitanism of an in fact extremely provincial Lombard town in 2024!
In fact a very tiny part of those will ever merry into local families, surely many will go back to their countries after some years or even decades.
Sure, demography could really change the genetic make-up over a long period of time but that is another matter we are not discussing here (although it would help to remind, in this respect, that today's genetic profile in central Italy is not the same as the "Imperial Roman" genetic profile either, suggesting to me a similar scenario to the one I've described in respect to my hometown).
My point is that a thorough knowledge of history is helpful for the scientists to interpret genetic data and I'm not sure I'm finding it in this paper. What I find though is the usual undertones pushing immigrationist (and in one particular instance even LGBTQ) agendas.