But presumably if you are better protected from getting infected in the first place with the MrNA vaccines, then a severe outcome is less likely too as a result? As you'd have to become infected first for it to develop into a severe illness
Except the stated protection is against "symptomatic infection", so we don't actually know if there is a reduction in infection with MRNA, it's always been a bit annoying that AZ and MrNA have recorded different things.
And if it is just high circulating antibodies, unless you're boosting every 3 months (I concede people will likely be) then the waning will be significant - Although the reported 70% reduction in infection is not good enough for anyone with large number of social engagements with the current prevalence avoiding infection anyway, even before it wanes. So I'd still say the higher protection against serious illness is what you need to go for.
Either way, I think the study is too small, with too many confounding things to say anything with assurance.