It doesn't matter which direction the ball is played as long as the player is behind the ball, which in this instance, Kane wasn't. His knee was in front of the ball based on the lines drawn
That's actually a very good point, and clarifies it for me somewhat.
I was actually confused as to why it had been disallowed, as the ball was played back, but in hindsight, I think I may be confusing the situation with incidents where the ball was played forward, but the receiving player was actually behind you! That's a different issue, but perhaps what was confusing me.
Your explanation (and I hope it's correct, lol) makes sense.
Another example could be a left footed player taking an inswinging free kick from the right side. The ball might immediately travel backwards, before curing forwards, but the player receiving it is still offside.
I think in last night's passage of play, the defender is irrelevant?