You are really underestimating how hard it is build and maintain such easy to use and secure services. So using a trusted service like Signal is convenient. And government officials across the world use it:
https://www.zdnet.com/article/in-encryption-push-senate-approves-signal-for-encrypted-messaging/
https://theprint.in/tech/netanyahu-zelenskyy-join-world-leaders-to-signal-each-other-why-is-encrypted-app-popular/1204419/
https://www.politico.eu/article/eu-commission-to-staff-switch-to-signal-messaging-app/

And moreover, the essay by the tankie creator of Lemmy does the mistake of assuming everything the US government funds or has funded at some point as nefarious. The US government is not unified by any stretch of the imagination. It is full of competing interests and some agencies do want to support ideas like freedom of information and right to privacy. If you look at the things that the Open Technology Fund has donated to, you will see that it has pretty much stuck to its objective of supporting “open technologies and communities that increase free expression, circumvent censorship, and obstruct repressive surveillance.”

And I still fail to see any real evidence for the claim that Signal’s privacy is compromised.