Agreed, but I think that majority of I2a1b absorption came after Slavic identity formed. I believe Slavic identity formed before Southern Slavic identity formed (which really should be a no-brainer), and that the vast majority of I2a1b absorption occurred between the formation of the original Slavic identity and the formation of the more recent Southern Slavic identity. How else do you explain the large discrepancy in I2a1b concentration between Southern Slavs and their Eastern cousins (who clearly predate the Southern Slavs)?