Then why did the CDC say in March 31st that it had just been confirmed?
The CDC has been indicating for a long time that they believed asymptomatic transmission was occurring. The only thing that changed on March 30th is that the CDC reported the results of a new study further confining that fact.
As early as January 31st, the CDC was relying on reports of asymptomatic transmission to guide quarantine recommendations. LINK
Here is an article from February 13th where one of the CDC directors says they are operating under the understanding that asymptomatic transmission is occurring based on data from China, just that the rate of that transmission was not yet fully understood: "Redfield said that an infected person not showing symptoms could still transmit the virus to someone else based on information from his colleagues in China. 'There’s been good communication with our colleagues to confirm asymptomatic infection, to confirm asymptomatic transmission, to be able to get a better handle on the clinical spectrum of illness in China. What we don’t know though is how much of the asymptomatic cases are driving transmission,' he told CNN’s Chief Medical Correspondent Dr. Sanjay Gupta in an interview on Thursday." LINK
Here is another article from a couple weeks before Kemp's statement stating: "CDC officials have said that while it's clear asymptomatic spread does happen, it does not appear to be the driver of the outbreak.'" LINK
More importantly, there was virtual consensus in the scientific community for weeks if not months on this point before Kemp made that statement.