If the CIA approves of Signal for use internally - particularly the use case of communicating with CIA personnel in the field - this likely means the NSA gave them a thumbs up. So this implies that bare minimum no other intelligence agency can defeat Signal's encryption. Does that mean the NSA can't either? Up for debate I guess, but quite likely no for them as well.
This doesn't mean the platform it is running on is secure blah blah blah, but hey, when you think about it this is a massive endorsement for the use of #signal
https://www.businessinsider.com/cia-head-signal-use-senate-hearing-the-atlantic-tulsi-gabbard-2025-3