There was a british study around a month ago that claimed that natural immunity was as good as vaccination. And isn't that how it usually goes? Genuine question. The data from Israel also offers conflicting views, where vaccinated people have had not too impressive protection. I'm not saying the vaccines are bad, but rather that they are comparable to natural immunity.
The tests are a game of chance, right? But you are correct, he very well could have contracted asymptomathic covid. But that's also very pointless. But yeah, it could also be a true positive, shouldn't have ruled that out.