I look at it this way, if it was reversed, and a forward is leaping for a mark, and the defender with no eyes on the ball does that to him ... It is a free kick every time!“Illegal shepherding” would be wrong, even if in the goal square, right? So, I believe it was paid “blocking the mark”.
Having said that, I don’t know if that call would be right either.
IMO it was the correct call by the umpire.