It's at least CUI. At _least_. I would suggest material that is operational in those regards would be secret or top secret. It had:

* timelines of kinetic resource missions * conops including order of battle, etc.

Per normal ODNI bits, it's pretty clearly within bounds for classification.

The problem isn't specifically the release of this information, which to my understanding didn't reduce the effectiveness of the operation. But the real problem is that now that other state actors and intelligence agencies know that our top military and intelligence officials sometimes share details of impending operations on their personal cell phones, there is now a pretty strong incentive to try to crack those phones or crack the Signal protocol somehow to get access to that information. And Signal and Apple do their best but they don't have the kind of resources that the NSA or CIA can bring to bear on securing a piece of equipment from espionage.