> I think they mean any symptoms.

I think so, too. The referenced article (https://www.thelancet.com/journals/lancet/article/PIIS0140-6...) says:

“Every year, in the USA, about 25 million people visit their family doctors with uncomplicated upper respiratory infections, and the common cold syndrome results in about 20 million days of absence from work and 22 million days of absence from school.”

That makes 44 million days of absence on a population of 340 million, or about one day for every 7½ citizens.

Even assuming healthy people are as susceptible to that as the general population and assuming 50% of those that get ill will work, anyways, out of financial necessity, that still is a far cry from “Healthy people spend roughly 15-25 days each year […] sick”

That paper also has a graph showing Mean annual incidence of respiratory illnesses by age group. Eyeballing it (ignoring size differences between age groups) I get at an average of about 3 infections. If each takes a week, that gets you to those 15-25 days.