I see, you're right. What I wrongly assumed was that they were receiving spotty data about overall cases but more thorough data about cases leading to hospitalization.

I also just noticed the age brackets for the 508 known hospitalizations only add up to about 91%, so apparently 9% of those are also unknown age.

It's too bad they didn't publish this in a live form so we could see how it changes as more data comes in. Maybe they will do a follow-up using the same methods at some point.