Even if you discount Signal he did more or less design the protocol that WhatsApp is using https://techcrunch.com/2014/11/18/end-to-end-for-everyone/

Also while we would expect heavy promotion for a trapped app from some agency it's also a very reasonable situation for a protocol/app that actually was secure.

You can of course never be sure but the fact that it's heavily promoted/used by people on both the whistleblowers, large corporations and multiple different National Officials at the same time is probably the best trustworthyness signal we can ever get for something like this.

(if all of these can trust it somewhaat it has to be a ridiculously deep conspiracy to not have leaked at least to some national security agency and forbidden to use(