Not sure if it is an illegal advantage.
That's how former umpire John Holder described it in the discussion on the Mankad in the most recent "Ask the Umpire" (part of the Test Match Special podcast).
I think you can back up but then you risk being mankaded. That's the risk a batter takes if they back up out of their crease before the ball has been bowled.
I've just checked the laws and the situation is specifically enumerated under law 41.16 which is in the section on unfair play. I think that is the basis under which John Holder called the non-strike batter being out of their ground "illegal". Maybe he could have said "unfair" instead.
Note that, as of the 1st October, 41.16 will be moved to section 38 which is all about run-outs. From that point on, the non strike batsman being out of their ground as the bowler bowls will no longer be considered unfair but just the same as any other possible run out.