Yup, that's the way it happened. When someone comes in 6 inches off the ground when the other player gets there and the other player either hurdles it or just gets lucky, that is a case of "serious injury if contact". When the player is already on the ground before the player gets there, sorry, that is the same risk every single tackle carries, as in, there might be contact and anything can happen, that is life, but the risk of a leg break or something really serious is very small.
Hit someone in the foot, unlikely to break leg, hit them 2 inches above the ankle, very good chance to break leg, its really that simple. ACtually I'd say, hit someone two inches above the ground, and all your weight in the tackle while off the floor, very very good chance to break the leg, on the floor, bent leg, with significantly less weight behind the tackle and there is a risk but its significantly less. That is why "lunging" and stamping tackles are being red carded so often, because even when there is no contact it is the risk of significant contact so getting players to not even consider those tackles is a good thing.
Again watch some replays of most tackles, most of the time at some stage on the way to the ground the players weight is "in the air", the difference between a lunging dangerous tackle, and a normal tackle, is where you are going to hit the ground, before the player gets there, or catch him before you hit the ground.
I mean, by the theory that all tackles are dangerous in that sense, if Nani was 10 metres away and Kompany did the same thing, still a red, what if Kompany was 40 metres away from anyone and jumped in to prevent the ball going out of play, what above Nani standing still 5 metres away. THe proximity of a player determines what is safe and what is not, always has done. You can do a ridiculously high footed kick on the ball if no one else is around, do it right by another players head, even without catching them, its simply dangerous play.