I2a1 isn’t definitely shown to be native to the Western Balkans. The major groups in Bosnia vary differently with it too. Last study shows that Bosnian Croats carry the marker at 71%, Bosniaks at 44%, and Serbs at 30%. If we consider just that marker then it’s clear that most Bosniaks share a more similar gene pool to Serbs than to Croats. It’s probably true that the majority of Bosniaks today are descendants of medieval Serbs with few being Croat or something else. There was a huge population influx into Bosnia during the Ottoman Empire from other regions conquered by the Ottomans. Most of the original medieval Bosnians were depopulated from Bosnia upon the Ottoman invasions and most fled to neighboring Hungary-Croatia. For example the highest frequency of the “Bosnjak” (i.e. Bosnian) surname happens in Croatia among Catholics.