Yes, it’s not that much of an error or complete miscarriage of justice, if indeed one at all. The assistant deems the corner kicker to be offside at the time the other player plays the ball.
I wasn't sure about that one at first but, on reflection, think the offside call was correct.
There was confusion about several of the game's laws, and I honestly don't know how they apply, especially as they have changed so much down the years.
The corner taker was in an offside position when the second player touched the ball.
You can't be offside if the ball is played backwards (but don't you have to be behind the ball for this to apply?). The ball was neither played backwards or forwards but was stopped stone dead, but this may not be relevant anyway.