The reason that the unknown ages were excluded from Figure 2 is because the hospitalization status is also unknown. You assumed that 0 of the unknown cases were hospitalized. Instead, you could also assume that hospitalization rates for the unknown cases are roughly the same as for the known cases. If you do that you actually get the higher of the 2 numbers in Figure 2's range. So for the 20-44 age group, that's 20.3%. If you instead assume that people with a known age, but an unknown hospitalization status were not hospitalized, then you get he lower number in the range, 14.3%.

It's a very fair point that any of these assumptions could be inflating the rates. There are a whole bunch of other factors around asymptomatic, what the eventual outcome will be in the cases that are early-stage and who gets the limited number of tests in the USA that makes it currently impossible to know the true infection fatality rate or infection hospitalization rate.