We are starting to go around in circles. The AR's call for Knock-on was incorrect, due to his poor positioning. The refereee would have been better off consulting the TMO, as he was entitled to do. Unfortunately he choose not to, since his assistant was "sure" of what he saw.
The other aspect was brought up as a filler by the TV at half time, awgthtgtta. The support runner was behind the ball when it was last played. The slow motion is "unhelpful" in the respect that it allows decisions to be based on fractions of a second. The ref cannot during the run of play disect each action to the nth degree. Ian is right the commentators have superimposed the line. It's correctly done, don't you know the first thing about perspective (vanishing point, etc.) ? The TMO had the images taken from in-goal available to him. Those would have been sufficient to tell the ref there had been no knock-on. If the (unasked at the time) question was "Is there any reason I cannot award the try?" then the potential off-side would have become an issue. Since it's marginal, & unclear from mutliple replays, he could only answer "There is no reason. You may award the try."