I have a hard time with that one.
You bring up a good point. Of course there are plenty of people who are ethnically (or "racially") and halachiclly (according to Torah law) Jewish because there was a Jewish woman somewhere down the female ancestral path. They might not even know it, but they are still Jews. But there is more to being Jewish than DNA. There is a religion called "Judaism" which "(racial) Jews for Jesus" do not practice or believe in.
If the "Jews for Jesus" could actually trace their ancestry in an unbroken progression all the way back to the days of Jesus himself, meticulous observing all the laws of Judaism and also Christianity, then they could legitimately claim to be a "Jewish sect." However, they are a gentile missionary society that targets the assimilated and the religiously illiterate. They have no schools or seminaries of their own, they get all their training from Christian evangelical institutions.
I thought I was specifically differentiating between the Jewish religion and the Jewish ethnicity. I could be wrong, but I thought that the one doesn't necessarily follow the other.
For instance, Sammy Davis, Jr. became a Jew. Therefore, was he in fact Jewish, or did that not count?