The must is emphasized to mean that it can not attack a non-flyer unless it loses the flying trait. If it is unable to do so, then it is unable to attack. Otherwise, the must could have been left out and they could have simply said that they do lose flying when attacking a non-flyer.
It would certainly be possible for them to change the Reach rule to allow the interaction you stated. That is partly the impetus for my question, to see if they had done so. Such a change would have several consequences on the game though, and so I doubt such a change is intended. I believe it was simply an oversight and that the previous rulings are still correct. Meaning that a flyer is not allowed to make an air + ground melee attack, regardless of reach.