Yes, it is.
Members are Jews from birth who choose to be Christians through faith in Jesus as Messiah.
Generally speaking, at least half of 'messianic' congregations are not Jewish by birth, but rather are gentiles who think believing in Jesus and attending a seder makes them Jewish.
Jews define who is a Jew. Someone who converts to another religion has cut himself off from the Jewish people.
That's not to say they can't return, but they would need to renounce their apostate beliefs first.
Let's see some proof of your statements.