I agree. There are official channels that already exist for discussing sensitive information, and it does not appear Signal is one of them. These officials using any device or software not approved for that purpose constitutes a serious breach of protocol.

Signal probably shouldn't be approved for that purpose because it does trade some foolproofness for convenience. Secure communication should also be limited to dedicated devices, which probably wouldn't have journalists stored in their contacts.

The CIA was approved to use signal but for certain applications. Probably because it was better than SMS. But not good enough for classified information.

You could see a CIA agent being in Russia needing to use Signal with an informant, e.g. But that wouldn't be the same level of security needed to hold nuclear secrets.

I imagine Signal itself is secure enough that it wouldn't be unreasonable for a government to develop a procedure to use it to transmit classified information under certain conditions.

That list of conditions would likely be quite restrictive compared to how we saw it used here. It would certainly include using a dedicated device for classified information, and would forbid taking that device to an unfriendly country. The US government doesn't need to do that though; it already has its own systems for secure communication.