Signal is the world’s most widely used truly private messaging app, and our cryptographic technologies provide extra layers of privacy beyond the Signal app itself. Since launching in 2013, the Signal Protocol—our end-to-end encryption technology—has become the de facto standard for private commu...
So, literal hearsay.
The server is centralized so it’s irrelevant whether it’s open source or not, we have no means of checking.
Seems you’re referring to initial funding from the Open Technology Fund. That’s a US government body that promotes technologies that undermine authoritarian regimes. Signal fits the bill perfectly. In any case that was a decade ago. Since then there has been far more money from various do-gooding individuals and foundations. In particular the Freedom of the Press Foundation, which (I just checked) is vouched for by various whistleblowers including Edward Snowden. So, hardly a stooge of US imperialism.