This is wrong empirically and providing proof for this is how Acemoglu and Johnson won the economics Nobel. In basically all maps of voting patterns within Europe you can read its institutional history. You can see the border of the Holy Roman Empire in economic and voter data in Poland, you can see the iron curtain in every map of Germany.
If you want one of the counterexamples to your Italy theory, Venice was one of the richest middle-age cities in Italy and it is famously built on water.
You really have to explain specifically what you mean by this phrase, or else it's typically just saying you don't actually understand the rule or the exception.
It sounds like you're claiming Rome succeeded for reasons that overcame its geographical disadvantages, and due to this growth protected itself from naval invasions. But Rome was not a maritime power during its early republic period, let alone earlier. So why didn't Carthage or anyone else just sail upriver (Rome was not on the coast, just to clarify the context) and destroy Rome? How did Rome succeed in the first place to become a maritime power capable of defying southern Italy's geography?
I would note that the north and south of Italy have very different geography and climate. Which can be upstream of all sorts of things, culturally. The geography of Italy's two halves support different types of economic activity; and the social realities of living within these different economies, naturally evolves into major differences in culture. (Compare/contrast: the differing cultures of coastal vs midwestern America. Now imagine that split with a few thousand more years for the divergence to take hold.)
History happens once; but geography is always affecting a nation, all throughout its evolution. So if you're looking for reasons that two sub-populations within a country might have noticeable differences today, differing geography is going to be the "horse", while history is more of the "zebra."
That being said: geography can also constrain history.
Southern Italy is almost entirely coastline, in a part of the world where, for much of the last ~2000 years, everyone was constantly invading everyone else by sea. Northern Italy was relatively-more immune to amphibious assault, as its capitals could be situated more inland. (Rome itself — the exception that proves the rule — was located in south Italy, but was defended from amphibious assault mostly by the Roman Empire's huge naval home-fleet being docked to the southern-Italian coast; not by anything inherent to its location. Once the Roman Empire itself went away, big rich cities in southern Italy suddenly became juicy targets for conquest and/or sacking.)