I believe the DoD can use applications in the JWICS suite of tools to communicate up to top secret level information. With the DCS platform for only Secret level communications.
However, these tools are on dedicated networks and dedicated machines. I suspect the reason for Signal usage here is that they were using their personal smartphones, which would never get on the JWICS or DCS networks.
Further, one of the members of the group chat was in Russia at the time, because he's the special envoy to Russia, which means that the communication was likely viewed visually by cameras if he were to open the signal chat on his phone.
It's hard to understand this level of incompetence from supposedly "senior" people that are running our government. I would hope that any company would fire people like this immediately. But because they are politicians, they are apparently unfirable.
(And this is entirely ignoring the only reason we know this is going on, that they added a journalist to the chat, one that they claim is particularly unreliable, further damaging the opsec credibility of the politicians.)
It was a confusing Apples Vs. Oranges comparison. A SCIF and Signal were contrasted with one another, like they were providing the same or similar facilities.
There are permissible chat applications that support Top Secret tier information which is actually akin to Signal (e.g. JWICS, DCS, platforms). A SCIF is a secure place, you still need communications technology to get information to and from different remote secure locations inc. SCIFs to even hold those meetings.
> one of the members of the group chat was in Russia at the time, because he's the special envoy to Russia, which means that the communication was likely viewed visually by cameras if he were to open the signal chat on his phone
One of the members of the Signalgate chat, Witkoff, was in Moscow meeting with Russian leader Vladimir Putin. Just after midnight Moscow time, the chat named an active CIA intelligence officer; according to a Telegram post by Sergei Markov, Witkoff and Putin were in a meeting until 1:30 a.m. [1]
[1] https://www.cbsnews.com/news/trump-envoy-steve-witkoff-signa...