Apparently that is despite clusters that may sometimes suggest otherwise.
Keeping in mind that a pandemic is largely about numbers - how many secondary infections occur in close or other contacts - and how far and fast that transmission chain continues (the yellow circles and beyond).
For me, this really reinforces just how important it is to have a full picture of a virus's transmission pathway. Not just the severe cases that show up in hospital, not just their contacts but also mild and asymptomatic cases in the community and the rest of the hospital. Prospective screening without regard to signs and symptoms in fact.
Each and every person positive for the virus may represent a link in the transmission chain.
You address whether mild cases can spread virus in another study. Oh, and we should probably keep monitoring all the viral strains we detect for genetic changes that occur in parallel with family clusters or upticks in transmission - which might signal increased potential to spread.
Don't test, don't find. Know nothing.