That is, Moderna is probably more effective, but people vaccinated with it have an even lower death rate because they're less likely to have been in a vulnerable group.

This is also a major factor that can be missed when going over the vaxxed vs unvaxxed death rates. If (eg) 75% of the population is vaxxed, 5% chose not to take it and 20% are too young (and thus very unlikely to die of Covid), then the difference in death rate will be much smaller than you'd expect based on vaccine effectiveness.

As a result, Germany e.g. uses half-doses for Moderna and vaccinates under 30s exclusively with Biontech.