The presence of antibodies isn’t as big of a signal as we once thought. There have been several companies developing antibody tests for various things, including your body’s own natural receptors. They have been appealing to people with chronic illnesses as an explanation for unexplained symptoms. For example, implying that if their tests show antibodies to certain receptors then that is a diagnostic clue. Then some…
1) this is a simple molecule, not an organism that can react to the threat. As you said, it would be interesting to be evaluate whether the antibodies actually have an effect, but they probably have, else they likely wouldn't have been detected at all.
2) the blood samples were taken from patients. It would be interesting to evaluate blood samples from the population at large.