Comment by Zak

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.