He's not sick. This is the new white lie to excuse oneself from attending events that are awkward or unwanted. It lets everyone off the hook without needing to produce the real excuse.
But stating the reason why you weren’t able to attend due to COVID sounds better than saying you have a case of diarrhea.
It's a very popular lie right now because no one will challenge it, not one word. People don't want to get canceled by the left.