This wasn't a double blind trial. This was a "Do something and watch the results" activity.

And consider this: "In these people, we were able to identify cells stimulated by a key antiviral defence response in both the nose and the blood. This response, called the “interferon” response, is one of the ways our bodies signal to our immune system to help fight off viruses and other infections. We were surprised to find that this response was detected in the blood before it was detected in the nose, suggesting that the immune response spreads from the nose very quickly."

This told us that one of our hypotheses about the virus was invalid. You don't need a lot of volunteers for that.

Smaller study sizes are ok if the effect is large.

I don't think your point is invalid; but as the siblings point out, sample size effectiveness really depends quite heavily on many factors. As an obvious/naive example, you wouldn't need a lot of participants to know that an acute poison is, well, poisonous. As you get ones with less of an acute effect, though, the number of participants needed goes up.

Similarly, they did controlled and deliberate exposures here.

So, I'm very excited to read follow ons to this study. Looks very promising to my naive eyes.