I even haven't read the paper yet, but considering what has been said here, I'd expect Italian population to be on a cline between Iranian mixed with EEF in the south and R1b-L51 mixed with EEF in the north, and I think R1b-L51 is steppe mixed with EEF in the Carpathian Basin ca 4.8 ka.
And it is true, after the fall of Rome, much of Italy has been depopulated and repopulated by immigrants, but these immigrants would in large parts be similar to the R1b-L51 mixed with EEF that were in Italy when Rome was founded.
My guess is that the Latins and Samnites that founded Rome entered Italy during the Urnfield expansion and were akin to the R1b-L51, steppe mixed with EEF in the Carpathian Basin.
I agree with some of what you're saying.
However, if I had to guess, the substructure in Italy is probably too precisely aligned with the groups in Italy before the Roman Empire for any massive effect from supposed "re-population" after the fall which just happens by chance to fall along the same cline that always existed. Northern slaves were sent to the south too, you know, and there were also Byzantines there.
That was supposed to be what happened to the North in the case of the Langobards, i.e. "massive" de-population during the Gothic Wars, and then the entrance of the Langobards changing everything. I always doubted that, given that population figures for those periods cannot be verified, but also because even if there was de-population, the Langobards, even according to their own records, numbered only 60,000 people. There would have had to have been no one left for them to make a "massive" impact. I think I may have been right, given that they seem to have carried U-106, and there's precious little of it in Italy, other than in the Northeast, and even there it's a very minority component. Even if you add in I1, this was not a major change in Northern Italy.
The Gallic tribes are perhaps a different story, especially if they carried some forms of U-152. For one thing, how different would they have been from the Italics in the first place? In the second, they arrived before 400 BC, which is when Ralph and Coop see foreign intrusion ending based on IBS analysis.
As for the south, I think it may have been like the north. Is it possible there was some intrusion by Byzantines who came as refugees? Yes, indeed it is. Enough to significantly change things? It's possible but not highly probable, I think.
Take another look at the two maps Jovialis posted.
Anyway, we'll know soon. I hope they consulted with people who know something of Italian history and pre-history.