If you’re willing to take extended data table 1 at face value [0], then perhaps 1/2 to 1/3 of the exposed cohort is in this category. This is at least moderately consistent with other data suggesting that at least around 2/3 of the naive population will contract COVID and test positive on PCR is exposed.

In other words, the authors found a population that apparently did clear the infection this quickly, and they didn’t scour a particular large cohort to find these people.

[0] which you shouldn’t.