You are assuming that what they found is indeed a marker of covid-19. It might well be that they found antibodies for another coronavirus that for whatever reason shows on the tests used. There are many coronavira and not all very well researched. Nor is each test precise in each condition, e.g. in the paper they describe how specificity and sensitivity vary depending on the age of the sample, then calculate false positive/negative values using one of those values. Has the test been tested for pregnant women and frozen samples? Might there be other factors, e.g. cross-contamination with some equipment they use?
Its not unusual for even well-done studies to later be revealed as flawed, so its important not to take results of one single study as ultimate truth.
Actually, what I tried to say is that prior to becoming a common disease, it was a rare disease for a while, and precisely because it wasn't common, expecting it to be found often is unreasonable. Saying that a find is unlikely because most people found nothing is meaningless, because we already know that the virus wasn't common before it became common.
I looked up the test they used, and it's among the better ones.
That's right, in the sense that a supercontagious disease cannot stay rare for long. For how long can is stay rare? For how many months? That's a nontrivial question for a diseases such as this, which has both superspreaders and a large number of patients that infect noone.
