beardy,
I don't see CDS walking into a trap at all; I think you are seeing what you want to see in his comments. Shortly after his comment that a Corbyn premiership would "worry him", he explained why. Bits of this have been posted earlier but I've added a couple more sentences for additional context:
‘But the reason I say this – and it’s not based on a personal thing at all, it’s purely based on the credibility of deterrence. The whole thing about deterrence rests on the credibility of its use. When people say you’re never going to use the deterrent, what I say is you use the deterrent you know every second of every minute of every day and the purpose of the deterrent is that you don’t have to use it because you successfully deter.’
All CDS was doing was explaining the doctrine of nuclear deterrence, a topic which is a staple of strategic studies textbooks and solidly in his area of professional expertise. He was not expressing a personal opinion in any way, or indeed trying to influence the party political debate. If the audience, equipped with a better understanding of deterrence, decided they didn't like Corbyn's policy, then does that mean CDS is guilty of political interference? I would argue not. Any democrat should be wary of arguments along the lines of "keep them ignorant".
Furthermore, since both Government and official Opposition policy are currently pro-deterrent, CDS can hardly be accused of entering a party political debate. Yes, there may be some disagreement within Labour at the present time, but Corbyn himself was complicit in keeping it off their conference agenda, so it is a bit rich for him to get worked up about it now. Can you imagine how little we would be able to say if we had to respect differences of opinion between each individual parliamentarian? Until Corbyn's views are adopted as Labour policies, that is exactly their status - individual differences of opinion. The fact he is leader matters not in Parliamentary terms.