Not sure why it's difficult for some to understand that Illyrians and Thracians mixed in the South-Central Balkans. Thraco-Illyrian is a real thing. Yes, Proto-Illyrians and Proto-Thracians started in different homelands, but by the time they expanded south, they would have naturally met somewhere along today's Serbia, Kosova, Northern Albania, N. Macedonia, Northern Greece. It's not shocking that they would be genetically similar by the Iron Age, maybe even sooner around the LBA collapse.
We don't know much about their languages, but some scholars and historians think they could have been as similar to eachother as Czech and Slovak, or Spanish and Portuguese. Some Illyrian and Thracian words are of the same root but have different endings.
Ancient groups like the Dardani, Bryges, Enchelae, Paeoni, etc, were recorded as both Illyrian and Thracian by different contemporary historians, and most modern historians say they were Thraco-Illyrians too. Given the location of these ancient groups in the South-Central Balkans, in between the Illyrian and Thracian spheres, and what historians have called them in antiquity, it seems that they definitely did intermix and were genetically similar. There are plenty of J2b2 and E-V13 samples that overlap or plot close to each other