They specifically corrected for that. Along with demographic corrections (age, income and geography via zip code, race), they found the rates of cough and fever for their sample versus the general population, found that their sample did have a slightly higher rate, and then reweighed for it. See page 23 in their paper.
I didn't see that in the abstract. But correcting for it isn't really the same thing. Otherwise you could just do any study as non-randomized and correct for it. The reason people don't think that is acceptable is you don't know what you don't know and you can't correct for everything.
And just having same amount of cough % isn't a good way to standardize anyway, because maybe they had it 4 weeks ago, or asymptomatic etc...
arn