The Novus Ordo Mass is held (by the old SSPX, +Lefebvre, +Williamson, etc.) to be "illicit but often valid, depending on the intention of the priest".
So let's take a typical Novus Ordo Mass, which is said by a valid priest reverently (no clowns, women in leotards, Barney costumes, Wonder bread, or any of those radical abuses).
We shouldn't attend because of the protestant elements that are contained, and because the liturgy is deficient. Over time, belief in things like purgatory, the Blessed Mother, the sacrificial nature of the Mass, etc. is destroyed because of the SINS OF OMISSION in the Novus Ordo.
But the Novus Ordo and the Conciliar Religion must have some Catholicism left, otherwise we wouldn't treat "converts" from that religion as already Catholic. But we do! When someone comes to Tradition from the Novus Ordo, there is no formal abjuration required (as there is with protestant converts), nor do they have to go through RCIA as catechumens before they can receive the Sacraments.
Now picture a young man, 18, with Downs Syndrome. He is a sweet guy, not too bright, but he wants to serve God. He believes that "going to Mass" is pleasing to God. Unfortunately, he was raised in the Novus Ordo.
Now this man isn't capable of understanding the Crisis. You can't talk theology to him. His heart is certainly in the right place, and if the Novus Ordo he attends is valid, then it could nourish his faith, right?
My point: there's not a whole lot of difference between a man with Downs Syndrome, a moron (a man with an IQ of 51 to 70), and an emotional, uneducated old lady. (Like the one in Mahopac, NY). In terms of ability to reason clearly, they are all equivalent!
So it's not that surprising that +Williamson would have one advice for THIS kind of people (who don't really have the use of reason), and a whole different set of advice for the rest of us, who do have the use of reason.