Point 5 makes no sense. The whole point in using a surrogate key is that a natural key is almost always flawed. There are no good reasons for the advise he gives for point 5, in fact it is a mistake to do what he says!
I think his point could have been made clearer, but he's really just saying that there will often be a mutable natural key that is captured in the modelled data, and that it makes sense to enforced constraints over that key if you know that the constraints exist.
