He's talking about the possibility that the babies are 100% genetic copies of mom. It is very rare, but it does happen where babies are clones of the mom. So if you breed a visual male to a normal female, there's a VERY small probability that the babies aren't really 100% hets, but 100% normals. He's just being overly cautious with how he breeds his animals which is fine. Not everyone chooses to do this because it is a very expensive way to save you butt in case the rare thing does happen.