The Mashiach will be traced back to King David, the tribe of Judah. This would not mean Jews as a Jew can come from any of the 12 tribes of Israel. It specifically means from the tribe of Judah. When the 10 tribes left the rulership of Shlomos, son, it says, Judah and Binyamin remained under Rechavam (M’lakhim Alef 12:23). So I understand then that these people who are the Mashiach’s relatives are alive today and they are from Judah?
The term “Jew” stems from the tribe of Judah, for most of the 12 tribes of Israel were lost in the Assyrian exile, and therefore, most of the People of Israel today are from Judah (who was much larger than the tribe of Binyamin), hence, we’re called generally: Jews. As a result, most Jews/Israelis/Israelites in the world today could be from the Davidic/Messianic family of Judah, except for the Kohanim, Levi’im, and Etheopian Jews (from the tribe of Dan), or children of converts on their father's side, who we know are not from Judah.