look closely

at the verses.
Peter was living as a gentile, plain as day. It even says
not living as Jew, very clear verse.
Gal 2:14 But when I saw that their conduct was not in step with the truth of the gospel, I said to
Cephas before them all, “If you, though a Jew,
live like a Gentile and
not like a Jew, how can you force the Gentiles to live like Jews?
Now we see how the other Jews were living as Gentiles too. Because for Peter to live as a gentile, then revert back to Judaism, and then for the other Jews to do as Peter did, to follow in his same footsteps, means that they had to be living as gentiles too, since they took the same course of action. "along with him", that would be along with Peter.
Gal 2:13 And the
rest of the Jews acted hypocritically
along with him, so that even Barnabas was led astray by their hypocrisy.
So, the Jewish Christians were all living as Gentiles, then Peter defected, then the rest, for all of them to do the same defection, means they were all doing the same thing before the defection, and that was living as gentiles. So they were not living out Judaism in Antioch.