OP,
His name is Corbyn, with a y. I don't like him, nor his ideology, but he is an elected representative of his constituents and elected leader of his party. I, personally, think he is wrong, but he has a mandate to speak publicly about this country's defense policy, CDS does not, his role is to enact it, not make it, he is a servant of the Crown's advisors, Parliament. CDS would have been much wiser to say nothing, the chances of Corbyn's view becoming policy are slim. If they do, he has no choice but to implement to them.
It's called democracy.