The premise is that most of what we today consider to be tribal and ethnic groupings from Classical writers aren’t really so. They’re “subjective” groupings based on the personal followers of a particular warband leader, or a particular grouping of war-leaders. It was only later that the cultural identity of these tribes emerged, resulting in linguistic and cultural differences that could be readily used to identify one tribe or the other.
To say that this has implications for tribalist Heathenry is an understatement. Definitely worth reading.