I don’t think Signal itself (as a protocol and encryption scheme) is an op. The app may be, though.

I do think intelligence agencies promote it because it doesn’t aggressively protect against side-channel attacks (like keyloggers and message exfiltration if you have physical access to the device).

I can’t completely discount the possibility that they may be paying Moxie to downplay the likelihood of things like backdoored keyboard apps. The way the app is designed necessitates a lot of user vigilance in order to protect your identity and communication (up to and including getting a fake phone number not tied to your real identity in any way) but that all depends on your threat model. Signal gets me away from Facebook, isn’t trivially intercepted like SMS, and is something other people will actually use, so I keep using it, I just don’t expect complete secrecy or anonymity when doing so.