Very little is certain in any aspect of genetics, but as I said before, the academicians and virtually everyone in the hobbyist community except people from the Balkans like Garrick and Miroslav believe that this particular lineage of I2a existed in more northern areas and became part of the Slavic speaking community there, only arriving in the Balkans with the Slavic migrations of early Medieval times.
It seems to an uninterested observer that people who have been pumped with "Slavic" propaganda since the 1800s and maybe even before just want to have it both ways. They want to be "Slavic", despite all the evidence that most of them are at least half and often more definitely not "Slavic", and at the same time they want to be "indigenous", so they go through these contortions trying to show that the only y marker which could possibly be "Slavic" is indigenous, which would mean that the Slavic languages are also "indigenous", which is patently absurd. For one thing, except for some ethnocentrist linguists from the Balkans who are mostly ignored, every linguist in the world knows that Slavic is a relatively recent language, and every archaeologist, anthropologist and geneticist in the world knows that there was a migration by Slavic speaking peoples into the Balkans after the fall of the Western Empire and the weakening of the Eastern Empire. It was just a slightly later later Barbarian invasion.
You can't just look at the arguments for this y lineage being "indigenous". You have to look at the whole pattern or story that is being sold. If this lineage is not "Slavic", where did all the "Slavic" ancestry come from? There's far too little R1a to account for it. So, it has to be "Slavic". It also has to be "indigenous" for some people, but that would require us to believe that it was already in the Balkans before the arrival of Slavic speaking peoples in the Middle Ages. In that case what the heck y dna did those people carry? Simple, for these type of people. We'll just pretend that all the archaeologists relying on data and all the historians relying on actual documents are wrong and that there was no "Slavic" migration.
The whole thing is utterly ridiculous. How even someone who has been brainwashed by an education system and a culture under the control of autocrats and fifty or more years behind the times can believe this illogical and a-scientific narrative is beyond me. Of course, this is the Balkans....