if the BB had bet say 100 out of turn, then the SB had moved all in, the BB could choose to fold..... but he would forfeit the 100 chips as they have already crossed the line, albeit out of turn.
In the example in this post, the BB just happened to bet out of turn for a larger amount - by saying all in, he has effectively moved his chips over the line, even though they haven"t physically moved - and as far as I understand it, they cannot be withdrawn.