I still thinking that it has to be the technique of raising the geese. Although muscovy ducks are kosher in Israel, they aren't here in North America. Neither muscovies nor moulards are, because a 19th Century rabbi from Germany or Eastern Europe didn't recognize or like them. Don't ask the...
I know, but it was on one of the Jewish sites that the Chief Rabbi of Israel declared it kosher for importation to Israel.
I do know there is kosher goose or duck proscuitto. I, too, suspect it's in what they're fed.