You're the one who should watch the Cinematic again. Thrall literally just stands by the anvil without even moving a muscle to help Saurfang but the second rogue makes his move and jumps on Thrall anyways.
The assassin went straight toward Thrall. Attacked him, disarmed him, and stayed on Thrall the entire time.The one had a direct jump onto Thrall, but didn't go for him, Thrall didn't even dodge, he kinda just threw some ash towards her and the direction she was aiming wasn't for Thrall even prior to the ash.