Stalking Wolf can't be played to the battlefield you are attacking? | tcgmode

I just watched [this video](https://www.youtube.com/watch?v=aDbKS_s40WA) of LotharHS and at 15:00, I don't get the ruling of Stalking Wolf at all. Why is it not legal to play the Wolf to the battlefield the poro was attacking? Is it different whether it is an Open Showdown or a Combat Showdown? C...