True but Croatia did de facto become a geographic and political entity by EU4 time. So even if there was no Croatian state, the idea of Croatia existed, and it does make sense for a nominal Croatian power (to avoid getting into debate of cultures, here im using EU4 cultures, as they are assigned) that would control the core lands to claim Croatian heritage. Really its very similar to the concept to Russia; you had the idea of Rus', a legitimate political, historical and geographic entity, and then you had a bunch of successor states all claiming the Rus' legacy; more than one still does to this day, one is even called Russia, even though Rus' has been gone for centuries.
Similarly when it comes to Serbia, by EU4 time, it was a distinctive geographic, political and historical entity. But all of that is something that only actual Serbs would refer to or claim in order to link the ruler claiming Serbia and legitimacy. A foreign ruler (especially a non Orthodox one) claiming Serbia would not have their legitimacy recognized - so claiming Serbian heritage would mean nothing, the nobility would never accept it. This is why i can only see Montenegro doing this, as during that era they were genuinely Serbian by political orientation. Bosnia had way too many problems of its own to do such a thing imo, sort of for the same reason why Polish Catholic king had no chance of being accepted as the ruler of Russia