But the main problem with that explanation, is that it's not 100% clear that he was offside, as you can see from the still they used, there is a motion blur on the ball, which would suggest that it's already in motion and the lines are being drawn after it's been kicked and not from the initial point of contact.
The referee did not signal for offside either and instead signalled for a foul, if they were indeed looking at offside, we should have seen it at the time and they should have drawn the lines before the decision was made. Not 20 mins later.
It was a farcical decision making process.