Great shot.
To my mind, the problem - this being a clear instance of it - is that 21st Century technology is being applied in relation to 19th century rules, and it doesn't quite fit.
The laws of football (Law 11) refer to offside in terms of being offside 'the moment the ball is played' - but when you have the ability to slow it down to the point where you can move it on at a speed of 25 frames per second, it becomes clear that 'the moment the ball is played' is actually an ongoing process.
When a player plays a pass, what happens is this. First, his foot makes contact with the ball. Then, the air inside the ball compresses momentarily on the impact. At this time, the player's foot is still in contact with the ball. Then the ball regains its normal shape. The player's foot is still in contact in the ball, moving in the direction of the pass. Then the ball travels on its way, finally ceasing to have contact with the player's foot. When you are using cameras with speeds of 20/25 frames a second, these actions will all told occupy two or three frames. In other words, the ball is in contact with the foot of the player making the pass for one, maybe two, maybe three frames. That is enough time for a player moving forward at speed to go from a position of being level to a position of being half a yard offside. If the defender is stepping up at the same time, it makes the offside margin even greater.
But what is the actual 'moment' that the ball is played? The rules do not say. Is it at the point where the foot of the player who makes the pass first comes into contact with the ball? Is it when the ball ceases to have contact? A player can be offside or not depending on which interpretation of the rules you apply. The law was drawn up at a time where it was technologically not possible to have the sort of debate we are having now, and 'the moment the ball is played' was clear enough for all. But now that we do have that technology, and now that we can slow it down to the exact point at which the through ball is played, we may have to be a bit more precise about what we mean by 'when the ball is played' - especially if VAR is introduced in this country.
In the real world of course, the linesman makes a judgment on a split-second basis and sometimes they will (judging their decisions with the benefit of slo-mo moving at 25 frames a second) they will get it wrong. If VAR is going to be introduced, the result may be to replace one contentious, fallible means of deciding if a player is offside with another.