I'd say they have different cultures for them. Look at the baltics: Pommeranian, Prussian, Lithuanian, Letgallian, Ugric Baltic, Finnish, all seperate cultures. This promises that the Balkans will be many-cultured, as it should be.
The existence of multiple Baltic cultures doesn't say anything about the Southslavs. CK1 had all those except the Pommeranians, but only one massive Southslavic blob.
The issue wasn't that Paradox actually liked conflating all the South Slavs with the Albanians and Romanians, or that it's actually harder to figure out whether a province/person should be Serb vs. Vlach then any other culture. The issue is that every debateable decision in that region is actually debated. Many decisions that non-Balkanish-types consider non-debateable are debated.
For example the exact ethnic status of Slavs in the Republic of Macedonia is extremely debatable. Serbs say their Serb. Bulgarians say they're Bulgarian. They tend not to say much, because as part of ending their war with their ethnic Albanian compatriots they agreed to be neither Serb nor Bulgarian but Macedonian. If Paradox comes down on one side or the other of this debate the other two sides will flock to these boards, obscure maps in hand, implying Paradox is just a cog in the anti-Bulgar/Serb/Macedonian conspiracy. If they make the mistake of siding against the Serbs they will encounter the word "Serbophobia."
It should tell you a lot about how reasonable debate is in that region that a) some idiot decided anti-Serb bias has historically been just as evil as Anti-Semitism, b) his countrymen take it seriously, and c) his case that Serbophobia is truly vile is undermined solely by the fact that the most prominent alternative is ethnic cleansing.
Compare this to what would happen if the Danes installed the game and discovered that all of Schleswig-Holstein was ethnic German according to Paradox, or the Dutch finding out that Paradox's historical researchers put the ethnic border with Germany two provinces west of where they think it should be. They'd mod it, maybe make a polite request for a change on the forum, and if an ethnic Dane or Dutchman was a major player in the inevitable Improvement Project he'd arrange for it to be changed.
The inclusion of Croatian seems to indicate Paradox has changed it's mind on these issues, and has concluded that a) Balkan issues are less emotional now then they were five years ago, so the forum debate won't get out of hand, b) they have enough historical evidence to win any debate on their decisions or c) the ban-hammer works.
But it could also indicate they've only decided to remove Croats from the famous Southslavic blob, but they still don't want to deal with the contentious issues of where Albanian, Bulgar, and Serb culture borders end.
Nick