I don’t think they’re arguing that it was offside. They seem to just be correcting you that the rule is not whether the ball is played backwards, but where the receiver is in relation to the ball.
Technically, a ball could be played backwards but the receiver starts in front of the ball but drops deeper to receive the pass, which would rightly be ruled offside.
Seems to me from the image with the lines that the move is onside as Mcateer is behind the ball as it’s played.