>then how is he separate? They're not ethnically Hebrews but they're to be treated the same.
I agree, I would even say that the foreigner isn't separate at all. While treated at equal, the Israelites and God himself still recognized his racial difference as verse 49 shows. He is "as" a native born, that's a simile.
This is the same situation as in the NT church where there is Jew not gentile.
Anyway, my point is that you're not providing an argument in favor of mixed marriage from this text.