Maybe that is the case with some small, insular sects of Orthodox Jews? Some of the groups among those often referred to as the "Ultra Orthodox" - I don't know.

Maybe that is the case with some small, insular sects of Orthodox Jews? Some of the groups among those often referred to as the "Ultra Orthodox" - I don't know. It is not aligned with the vast majority of Jewish views about conversion to Judaism. In general, conversion is possible. As far as I can tell there are even clearly-enough defined requirements. Jews are not supposed to treat converts any differently than non-converts. People being people, this doesn't always happen, but that is the reasonable principle. Given that Jews for centuries have not proselytized to non-Jews, many believe you cannot convert to Judaism. You can.