Earlier quoted context omitted.
It makes a lot more sense if you don't assume from the start these people have one iota of intellectual horsepower. Signal is approved for government uses, just not non-public DOD information. They're supposed to use Signal for something like "hey, get to a SCIF so we can discuss details," then they discuss the details in a secure environment.
> Signal is approved for government use [Ref. needed]
Not approved for non-public DOD information: https://dodcio.defense.gov/Portals/0/Documents/Library/Memo-...