I agree with Mindbender - they know exactly who launched this missile.
Of course they know who launched it.
If it was ours they don’t want to talk about it for some reason.
If it was not ours it was pretty much an act of war so they either play dumb or retaliate.
The lack of anti submarine activity may mean it was ours or it may mean that we have already retaliated against the foreign submarine that launched it.
If so, the foreign submarine that launched may have already been destroyed which is also something we may not wish to talk about