Looking at PCAs can cause confusion, admixture paints a more complicated picture.

Northern Italians were ethnic Imperial Romans who were north shifted by an additional Northern European admixture from the Barbarian invasions of Goths, Lombards, and Burgundians and maybe immigration during the Holy Roman Empire. They have about half as much East Mediterranean but twice as much Northern European as South Italians who were still affected but to a lesser degree. The late antiquity samples from Urbino Bivio are from near Northern Italy but cluster with modern South Italians. I believe this is probably representative of Northern Italy before the incursion of the Germanic tribes if not more south shifted due to a little more East Med and a lack of a Celtic influence. The very large East Med admixture in them would be impossible to explain otherwise.

Nobody would ever regard a 1/2 Spaniard (most similar to ancient italics), 1/4 Greek Islander (east med), 1/4th Dutch (germanic) person to be Celtic or Germanic. Which is more or less what a Northern Italian is if we're going by closest modern ethnicities to ancient ones even though I hate these kinds of comparisons. You can't regard a North Italian as Germanic, Italic, Roman, Celtic, they are mixed between a lot of different ancestry sources from all over the place but are some of the most historically accomplished people in the history of Europe and the world which is why many want to claim them. And South Italians have the same building blocks but with much different proportions of these same components, except for Sicily which has some North African influence. Italians just don't neatly fit in with any other European group.