This is the old British-Israelism theory later reinvented by HW Armstrong and the old Worldwide Church of God.
News Flash: The ten tribes of Israel we never "lost".
Yes, I know they were never lost. I put quotes around "lost" because that's the way people refer to them, even though it is in error. It could be that Israel still exist as a nation to fulfill certain promises made to him even though he as fallen from grace, but spiritually the Church is Israel, not physically.
By the way what do you think about that "old British-Israelism" theory?