This reminds me of the evening halacha shiur at Neve Yerushalayim 20 years ago, taught by Rabbi Moshe Dombey ZT”L.
He prefaced the series about basar’v’chalav by pointing out that kashrus is one of the few areas of halacha in which, the less someone knows, the stricter he or she will be.
It seems that the person who originally told you that you can’t mix beef with chicken (or veal or whatever) got SOMETHING wrong, but it seemed right to him.