But you don't need to sit them on the shelf recharging, so you don't need to have many extra hydrogen modules (vs your fleet size). So, compared to dedicated hydrogen powered trucks, you'd maybe end up having 110 modules for every 100 trucks, for a modest increase in those costly fuel cells.

Maybe the swapping/charging stations could support both kinds and would prefer to give you charged battery modules, but if running behind in the charging, would switch to giving out hydrogen modules. Or maybe only some routes would have hydrogen equipped cargo stations at both endpoints, so the trucks would get different kinds of modules depending on the route - making a gradual rollout of hydrogen fueling infra easy.