The origin of not eating fish with milk is definitely NOT a typo. It is an explicit Beis Yosef, and there cannot be any question that he meant what he wrote when he wrote it. The question is what is source could possibly have been, and whether he was right.
Almost all Ashkenazim rely on the Ramo, who says that the Beis Yosef made a mistake, נתחלף לו בשר בחלב; he momentarily confused the danger of fish and meat with fish and milk. Therefore there is no problem.
Sefardim and a few Ashkenazim reject this possibility and say the Beis Yosef must have had a source, and he meant that just as it is dangerous to mix fish with meat it is also dangerous to mix it with milk.
However, since no other source is known, they tend to limit it to the BY’s words and avoid mixing fish only with actual milk, or things close to milk, but not cheese or butter. Thus many of them permit cream cheese and salmon.