The bible says a lot of things that Christian churches do not teach. Quoting anything in the old testament you might as well just say that Christianity teaches that you shouldn’t have long hair or ear piercings, which is clearly not true. That verse also does not say what you think it does. It says that you should watch out for people who say you “must” be circumcised to be saved. I’ve never heard anyone say that anywhere. That doesn’t mean it’s not encouraged in the church. That doesn’t mean it’s not taught. That doesn’t mean it’s not done as part of baptism (as it has been at literally every christian church I’ve seen). But yeah, of course no one is going around saying you won’t be saved unless you do so. That doesn’t make it “NOT a Christian thing”.