I don't think that you can infer your conclusion from his text. All he said (at least from the page you linked to) is that other nations begin to imitate us after the Cherem.
That statement doesn't exclude the possibility that prior to Jews, some other culture had some ban, formal or informal, but no one followed that ruling. And, more likely, numerous cultures probably practiced monogamy without specifically issuing a ban. Many cultures, historically, as well as today, see the advantages of monogamy, or, rather, saw huge disadvantages and problems with polygamy. (Personally, I've always wondered how the wise King Solomon managed with all those wives. Ah yes, Kohelet kind of explains things :-)