Jewishness by matrilineal descent was a later Rabbinic innovation, probably around the third century. In Paul’s day it was still patrilineal. Even today your tribe is patrilineal.

I’m confused by your question. Paul was from Tarsus, a Roman citizen, and brought Christianity to the gentiles/goyim, but was himself a Jew, from a Jewish family, and of long Jewish descent. Maybe I’m not understanding the implications in GP that you’re seeing? But I’m interested in seeing what you’re seeing.

It's not an argument, it's just a definitional thing, and I don't know much about Christianity or Paul but I don't see why it should have to "account" for him.