Weird. Are you sure the tweets talk about the same study?

The study in the article didn’t look at antibody response, but at the clinical outcome - just like the original Phase III studies did, just with a smaller group of people.

As for the claim that we don’t know how the vaccine protects from hospitalisations - if there is literally no difference in mild cases, why would there be any difference in severe?

Is there a single vaccine for any disease that offers no protection against mild cases, but a high protection from severe?