This technology behind this is very cool - I hadn't heard of optical frequency combs before.

My amateur interpretation is that the device shines light at a breath sample, capturing the signatures of the molecules in the sample (kind of like mass spectrometry), and the researchers trained an ML algorithm to classify the signatures as "COVID" and "Not COVID." For the ML part, the training set was n=140 and the testing set was n=30

I think the point of this study was to see if this approach was even possible, i.e. a proof-of-concept. They have a lot of work ahead of them to see if this could become a robust enough system to be used widely.