The only way you could accept Alito missed this flag, is if he was not home during that period (and nobody informed him).
Since he was so quick to throw his wife under the bus, he would have surely said so if that were the case.
But all that is to miss that the flag is a symbol of the politics within his household. It only highlights the bias, whether he knew it was being highlighted so publicly or not.
If I were a journalist, I’d be looking for records indicating his whereabouts.