He was injured in the middle of the field during a normal (for football) play. The Guard came at him, then the FB and then the ball carrier.
In the AFL the boards are made out of 10-15 inch foam before the actual boards.
This is just a freak accident which could have happened just as easily in the NFL.
I can't bring myself to watch the replay, but from the verbal description, you are right, the boards had nothing to do with it.
People are killed playing football all the time. It is only a matter of time before this happens in the NFL.