Wolf, you are saying two things: One, that the premise is not true, and there were other week lengths that were held. And two, that even if the week were universal that would be because the prevalent one won out.
To your first point, which is not only yours, he probably was not aware of such a group. Even if he was aware, if it is a unique and small group then the argument remains, since it is obvious that they tried introducing something new.
As to your second point, the one I addressed earlier, you are making an assumption that one system has to rule out, and I showed you that it is not at all the case. We get along pretty fine with different calendars. Our countrymen don’t even have moon months, and other countries have their holidays showing up at all seasons of the year. Nothing has to rule out. In fact, before the globalization that we have there was less reason to have a uniform system. Second of all, his argment was that it is the same on all parts of the world, which rules out the idea that one conformed to another.