You'd be surprised how many people omit this piece when talking of that stage, since it's that obvious.
Maybe this is what she meant, but the way she implies that they originally thought it was something else does not seem that way.
Right after purim, he was feeling sick for several days, but was sure that he hadn't been exposed. After Pesach, he found out that ALL of the boys he had gone around with had tested positive right after Purim.
Bottom line is that all of the info we have is that he was possibly exposed since people he was with tested positive and they presumed they had it on Purim too but there is no real indication that he actually had it including a negative antibody test while the rest of the family had high levels. Doesn't sound like it can be assumed with a high level of confidence.