When the specific bit of fiction was added to the book of fiction seems entirely irrelevant when it is the compiled book, including the later bit of fiction, upon which modern people claim to be basing their moral philosophy. I don’t believe the vast majority are reaching that verse and going “oh well this was added late so let’s skip over this part.” “Legitimate” (feels a funny concept for this topic, tbh) or not, it is included in most modern Christian’s interpretation of Christ