There was no exat lines. River Cetina wasn`t border line between "Serbian" or "Croatian" tribes. River Cetina was in one part of history the border line between the two realms, Croat one, and Serb one. This doesn`t mean that the old Neretljani and other sklavinije south from that line were "serbian" tribes. They were just vassals of Serbian king. Car Dušan ruled upon some Greeks, and they were Greeks not Serbs. History in this question is exact. You have core states of Croatia and Serbia, and surrounding sklavinije which acknowledge superior power of Croats or Serbs. During some time that sklavinije were incorporated in ethnicum of Serbs or Croats.

I, in fact, pointed out those lines are debatable, meaning I myself cannot prove they're exact. I also said Serbian and Croatian tribes, not Serbs or Croats, as these are ethnic designations of today. It may be confusing but if you look at the maps, only two Slavic tribal families are recorded on the Balkans and those are Serbian and Croatian tribes. Each of these tribal families consisted of more smaller tribes. This can be seen in the early Serbian states that were divided for a period of time after settling the areas. Nonetheless, these states are considered to be a part of the greater Serbian branch of the Slavs, at least in the Serbian circles, while many Croats consider these states as part of their Croatian tribal family, e.g. Bosnia, Paganija, Zahumlje, Travunija, and in some cases even Zeta, as I've read claims somewhere about Croatian tribes settling the entire coast down to north Albania.
If these states had preserved their identity through the centuries, no doubt the people living there would have started their own nationalistic movements, however, they were absorbed by their larger neighbors and therefore incorporated into their nationalistic societies. Likewise, Montenegro has every right to form their own nation, but to say they are not related to Serbs is a ridiculous claim. One does not need to renounce ties to other countries in order to form their own nationalist movement. We can see this among today's Bosniaks who would rather lie about some mythical Bosnian population of medieval ages (some claim even the oldest among the peoples of Balkans ::)) than embrace their past as it is and continue on a separate path if they choose so.