All of these guys have SCIFs at their homes and offices, and if they're traveling they have access to a SCIF. They can absolutely get secure messages across agencies in real time - Waltz even says this in the group chat when he tells them to check their "high side inboxes". They just can't get them as texts to their personal phones - for very good reason. It's a more inconvenient system by design.
What these guys are arguing is that there's a case for using Signal for something like what Waltz was initially doing - telling people to check a more secure system and asking them to name a point person. But of course the risk is that a) even that information is extremely useful to an adversary; b) once the more convenient system exists, you're relying on people to carefully adhere to the rules about what should go on it, and guys like Hegseth are morons who don't feel like they need to follow the rules.
What these guys are arguing is that there's a case for using Signal for something like what Waltz was initially doing - telling people to check a more secure system and asking them to name a point person. But of course the risk is that a) even that information is extremely useful to an adversary; b) once the more convenient system exists, you're relying on people to carefully adhere to the rules about what should go on it, and guys like Hegseth are morons who don't feel like they need to follow the rules.