Probably just an omission but here is an example of how hard it's going to be to keep track of the hidden gene. You can still produce normal normals (i.e. not hidden gene at all) unless one of the parents is platy or homozygous hidden. So the normal looking babies from hidden (aka normal sibling) X lesser breeding would only be 50% chance hidden carriers.
RDR did produce a normal looking female from platy X platy a few years back that should have been a 100% homozygous hidden so you can even have percentage possible homozygous with this normal looking mutation. Early on RDR did a lot of hidden X hidden (the het version offspring of the original platy) that produced a bunch of 25% chance homozygous hidden babies that would also be 50% chance regular het hiddens like their parents and only 25% chance real normals.