NC, where did you get the idea that I thought “achronim never hold that something in the S”A is a copier’s mistake?”
But this is definitely NOT a copier’s mistake, or a typo, and there is no such “shita” that it is. The Ramo certainly does not say such a thing. There can be no doubt whatsoever that the Beis Yosef himself wrote it, and that he meant what he wrote when he wrote it. He didn’t simply write “milk” while thinking “meat”; he meant “milk”. The Ramo doesn’t call it a typo, he calls it a “thinko”. He says that the BY confused milk with meat; that he misremembered the danger of fish with meat, and temporarily thought it was about fish with milk.