I can see now that it's possible the Sorbs spread their namesake to the south slavs already present, even though they were more genetically close to the Croats.
Possible, but I find it unlikely given that those already present don't really appreciate the new guy taking charge and choosing the team name (especially given that the new guy numbers 10x less than those already present (which wouldn't be the case if Sorbs derived from Serbs)). And if they were that influential, why couldn't they influence the Croats to change their name as well, given that they were closer to each other than Sorbs to Bosnian Serbs
In the same way, it's just as likely--using that line of reasoning--that the Sorbs from the north got their name from the Serbs in the south