He had to deal with the post guy though, he can't just stand there and let him do what he wants. So anything else is hypothetical. Had the guy not been there he would have saved it, but he was there.
Post man Giles could have stopped it too, never mind the two free defenders McNair and Akpom who didn't put enough pressure on. It's hard to do that mind, as you can either pick to follow the man or attack the space/ ball as defender, but neither of them really did anything.
Look how free the man is though, there's literally nobody within 5 yards.
View attachment 56469
Not saying Steffen doesn't have any blame, but it's 60% well played by them (combo of the ball in, the run, the header and the guy bothering Steffen), 30% defence and 10% Steffen, thus attributing least blame to Steffen.