Croatian here. It's not the pipes, it's just nonsense. Tap water is perfectly drinkable in Croatia and, as far as I'm aware, of great quality.
I've also traveled and stayed in Hungary, Romania and Bulgaria (in addition to already mentioned Serbia, Bosnia & Herzegovina and Montenegro) and no one mentioned anything about not drinking tap water.
The link you provided seems to be very unreliable source of information.
> Food and water standards in Croatia are similar to those in the United States. Most travelers do not need to take special food or water precautions beyond what they normally do at home.
So no, it's not based on CDC.
Both maps are just wrong. The reality is, 70% of global population has drinkable tap water, here's the data: https://data.unicef.org/topic/water-and-sanitation/drinking-...
Looks like unsafe dominates safe to me, by far. In the America’s, only Canada and the USA have potable water. In Asia, it’s only South Korea, Japan, and the rich city states (HK, Singapore, Brunei). In Africa, nothing, in the Middle East, just Israel.
Europe is the exception, but then it’s mostly just the EU countries. Serbia and Croatia aren’t listed as having safe drinking water (I assume it’s the pipes that are of concern, not source, like first tier Chinese cities).