Signal is end-to-end encrypted. One end is the Signal app on your phone. The other end is the Signal app on their phone. The Signal app is developed by people, using computers. Both of those things can be compromised, neither of them are under the perview of the U.S. security agencies.

I would put the market value of a backdoor into all Senior White House communications as certainly >$10B, and probably >$100B, limited only by how long the buyer believed it would be a reliable source of intel. (it may be better to offer it as a subscription service.)

At that point everything should be assumed to be compromised until demonstrated to a reasonable degree of confidence that it's probably safe. A random install from an app store is not that.