I'm not seeing anywhere that he actually claims someone doesn't have a right to exist. I see lots of people using colorful, subjective interpretations of other things he said to try and push a narrative, but that's all.
He didn't call for ethnic cleansing.
He did. He claimed that the demographic has changed and he advocates for reverting this change. And it turns out that forcibly altering the demographic of an era is the literal definition of ethnic cleansing (and by the way the phrass has historicallt been coined with the specific meaning of removing Muslim people from Serbia, by the people who intended to perform that “cleansing”, so it's particularly apt here).