Perhaps what Biden was trying to say is that Delta likely causes more severe disease in general, and so may inflict more severe disease on young people in absolute terms, even if they're still, ceteris paribus, relatively more protected from it than older people.
Nature has an article on the CureVac disappointment.
https://www.nature.com/articles/d41586-021-01661-0?utm_source=Nature+Briefing&utm_campaign=a2a2c7317d-briefing-dy-20210621&utm_medium=email&utm_term=0_c9dfd39373-a2a2c7317d-45428970
There are vaccinologists who agree with me that the unsubstituted uridine of the CureVac mRNA is a plausible reason, but it isn't the only one.
Perhaps what Biden was trying to say is that Delta likely causes more severe disease in general, and so may inflict more severe disease on young people in absolute terms, even if they're still, ceteris paribus, relatively more protected from it than older people.