If someone jumps over the ruck at you, you definitely can stand your ground. You are treating the "tackle in the air" and "don't jump into tackles" laws as complete absolutes but they aren't, they are down to context. If a player dives at your feet for a tap tackle and you hop over it, do you think that is illegal? If not then we have established you can jump to avoid being tackled and the question becomes in what context can you not do that.
The not jumping into a tackle provision is generally interpreted as jumping into a tackle (ie if the tackling player literally stood there and did nothing, you would create a dangerous situation). It's to avoid the person jumping from putting themselves and the person in front at danger largely through the chance of flipping over and landing badly.
Currently they err more on the side of penalising a player who tackles someone rather than the person jumping but quite tellingly, not one player who has attempted to hit a player diving for the line has been penalised for tackling a player in the air, no matter how high they have jumped. Many players have gone in to the air to a similar height to May and been hit into touch, the tackler was never penalised.