I don’t think they can tell if someone is contagious, or how contagious they are. I suppose you could try to measure something like number of viral particles shed per minute of speech. Sounds more like research work than a clinical test, though.Had - as in sometime in the past - or have a currently active case that could spread to others? Can they determine that from the test?
Even then, I think you’d find that the 2 infected passengers were shedding a small amount of virus. Not enough to ring alarm bells. But enough that you don’t want them to be in a closed environment with 2000 other people, either.