> "The vast majority of people infected with Covid-19, between 50 and 75%, are completely asymptomatic but represent a formidable source of contagion"

So long as every asymptomatic person got it from someone with symptoms, they necessarily a source of contagion. Is the conclusion based on modeling, i.e. that the rate of spread is only consistent with the asymptomatic people also trasmitting the virus?

Lots of asymptomatic infected doesn't (automatically) mean lots of transmission from asymptomatic people to uninfected people.