I wonder if it is just organizations that don't really care about anything other than brand name (signal is known as pretty good, right) and CYA.
Like it might legitimately be the case that you personally have expended more brainpower trying to understand the decision than they put into making it.
Or there might be an issue with trusting their own IT departments. With Signal they don't even have to trust Signal (haha, but they might think that you know).
There's another possibility: NSA told them to use Signal w/ TeleMessage so that NSA could see everything because they have an agreement with TeleMessage or because NSA knows about all these vulns in TeleMessage.
Like it might legitimately be the case that you personally have expended more brainpower trying to understand the decision than they put into making it.