Is there an official statement of this on a government website?

The natural and insider language of the chat, and (especially) the perfect timing of the strikes with the planning in the tread, also make it extremely unlikely this was anything but a genuine conversation, even without confirmation. The alternative is a combination of a very-prepared fraudster with either their own source of privileged information (to get the timing right) or else an incredible coincidence such that their entirely fake and uninformed planning matched the timing set out in the real planning. That it was genuine is far, far more likely than either of those (one of which raises its own, different security concerns, anyway)