I have reviewed the incident several times in slow mo where Umaga put in the kick at 69:05 on the match clock.
It is obvious that Zac is onside. Umaga is 2 or 3 meters beyond the 5m line while Zac is behind it by quite a bit 15m from touch.
From the camera behind you can see the ball lands in the middle of the tramlines 2.5m from touch and bounces directly upwards and Zac catches that far from the touchline. So unquestionably the kick was spot on.
The Leicester player is covering across from about 10m in front of Zac and about 10m further infield, giving him an advantage of distance to where the ball lands. If the ball lands further from the touchline he will probably get it first and would be 1 on 1 to get a try himself.
Zac swerves nearer to the touchline on catching the ball to evade the Leicester player and nearly succeeds. However, his left foot just clips the touchline by a couple of cms a split second before the Leicester player makes the tap tackle and the touch judge, who is right behind the action, raises his flag.
Hope that clears up any confusion or misconceptions.