[Sanzig]

Not to mention plenty of DoD facilities from coast-to-coast with SCIFs - even without a portable SCIF, he likely wasn't far from one.

Failing that, these people almost certainly have laptops connected to DoD networks at a lower COMSEC level than a true SCIF (indeed, "high-side laptops" were mentioned in the Signal thread). They could have communicated with those. I don't know about DoD policy if those would be acceptable or not for discussions about planned strikes, but it'd be a hell of a lot more secure than unsecured public smartphones.