Annulment does not end a marriage. It is not an analogue to divorce. Robertson was speaking of divorce, which, in his view, does end a marriage and permit remarriage. Annulment declares no sacramental marriage ever existed.
Alzheimers cannot be a basis for annulment unless someone had advanced Alzheimers at the time of marriage. This is next to impossible at normal ages of marriage. If a couple in their 50s, 60s, 70s, 80s tried to marry and one of them was in advanced Alzheimers, then they should never have been issued a license by the state nor should family or clergy have let the “marriage” happen. If undiagnosed Alzheimers existed in a, say, 70-year-old at the time of the marriage, then it wouold have to be demonstrated that he or she did not have sufficient sound mind to make a promise. But if that’s NOT what Robertson was talking about. He was talking about someone who married sound of mind and long years later gradually reached the point of advanced Alzheimers.
There is no analogy between what Robertson said and Catholic teaching on annulment. None. Nada. Zero. Zilch.
I guess I needed to have pulled out an hour or two of research in Cannon Law & the Catachism before I launched. Still, my understanding is that this tragic condition supplies the grounds for annulment, dissolution of the marriage. I appreciate correction and want to be correct.