They do justify their choice in the actual paper (e.g. see the end of the "Study population" section).
(It's also interesting to compare COVID hospitalizations to other ones, say from a heart attack or allergic reaction, but those are different questions. For example, maybe a COVID hospitalization is less bad than one due to a heart attack, or worse than an allergy. Or maybe any hospital visit is just bad period. But given many people got hospitalized by COVID this year - something that never happened in the past - we need to know what that event cost them, that's the point of this study.)