That challenge should be ignored. It's Lawyer bait and thinly disguised.
You may be half right however. If we consider for example, how it would look for Cardinal George Pell to 'defrock' himself during a Royal commission. An assumed admission of guilt would apply as would any other person holding a senior role in The Bureaucracy. Threatening to sack that same person before the results are in would put the 'ace' firmly in the wielders hand. Reverse psychology and all that.
Also it may be advantageous that he stay so the wielder can use his 'ace' against him thus protecting his bum and making the person accountable for his actions or non actions. The word scapegoat comes to mind.
At present I can't see why the DAS would want to hang around any longer than he has to, thus giving me the impression there is purpose to his extension of tenure for such a short period.
But I was wrong once before.