It was from another member of the forum, although I cannot cite the thread. The gist was that he used to play with Raging Barbs on until a game where he landed on another where there was only one AI, and he was faced with many Lvl 6 units. Since I don't know in what thread I read that, I can't cite it or double-check his story.
However, now that you have expressed doubts, something has occurred to me: his conclusion was based on the assumption that there was always only one AI on that continent and so the only experience source would be the various barb cities the AI was slowly conquering. What if the AI civ was the only surviving AI civ on that continent, and it had destroyed at least one other AI before the Human discovered that continent? Then the Lvl 6 units would have been veterans from AI vs AI and not AI vs barbs.