Watching the replays I don't see how it's missed. He steps ahead of the defensive line well before the ball is kicked.
Right he stepped early, but he was back in an onside position shortly after the ball was kicked from Kane in the middle of the line. It's physically impossible for the linesman to 100% accurately judge when the kick is taken with absolute certainty. They get it right a lot, but they also get it wrong a lot.
It's clearly offside, but the issue is that you humanly can't judge that to a high degree of accuracy, especially in cases of players moving at significantly different paces in a line, and that's why VAR is needed.
In other news, the idea that Alli wasn't marking him because he was playing him offside is not remotely true. He wasn't marking him because he got picked and didn't try to get good position before the kick was taken. Also the more crazy marking was the outside Spurs defender (whoever it was) playing on the outside of the field to mark the wide Juventus player for some strange reason instead of covering the end of the line.