Honestly its common sense. 80% of who they say have it show very mild symptoms. Not hard to assume those people are not getting tested with the limited availability of tests.
https://www.who.int/docs/default-source/coronaviruse/situation-reports/20200301-sitrep-41-covid-19.pdf?sfvrsn=6768306d_2
https://www.businessinsider.com/what-coronavirus-mild-symptoms-are-fever-2020-3
Yes.