He didn't make a move for the ball but he impeded the defender - it's interfering with play.
Same as if a striker is stood offside in front of the keeper, he might be standing perfectly still and not going for the ball but he's still interfering with play.
Why did the entire defence just part then like the red fucking sea and allow a free shot from the edge of the box? Lucky it was a wank shot but he had so much time and space! Should be 2-1 down