The original location is a matter of historiography and in Wiki article about White Croats we can find more theses. But the primary source itself DAI, clearly says that the ruler of Zachlumia is from the Vistula. And Historia Salonitana(13th century) talks about Croats which coming from Poland.
You can believe whatever you want but the statement 'DAI clearly says that the rurler of Zachlumia is from the Vistula' seems false to me. What is your source?