mbachur-
It’s halacha because the gemara says it. All I said is that in general no rishon or acharon would have the pleitzus to argue on anything that is mefurash in the gemara without a machlokes, so one can assume that such a thing is halacha because the gemara says so, regardless of whether or not the SA mentions it. And it happens to be the SA does bring down this halacha, I just didn’t have a chance to look it up yet.