Just guessing but perhaps what they meant was that Signal allows one to invite anyone into a chat thread whereas their actual in-house classified comms will not permit that without going through a massive chain of approvals and being assigned custom hardware.
That, and it's vulnerable to the usual sort of attacks that governments are capable of.
If your threat model is "local cops" or "nosy people" then Signal seems very secure. If your threat model is "Enemies of the US" then honestly... nothing short of a SCIF is going to cut it.
True and even a SCIF is not foolproof. From the 50's to the 90's the US and Soviet military played a lot of fun games one of which included typing up a classified document in a SCIF and the Russians met up with the contest judge with an exact replica of the document. There were thousands of such games. I learned about many of them in the military. Between those stories and the fact we bought transmitter parts from them during the cold war showed me that each nations military were never enemies, just the bureaucrats were.
