Thanks to @micahflee we can all examine the source code of that weird #Signal app variant from Telemessage called TM SGNL, that is used by Mike Waltz and possibly more Trump admin members, conveniently on GitHub.
iOS: https://github.com/micahflee/TM-SGNL-iOS
Android: https://github.com/micahflee/TM-SGNL-Android
(it's GPLv3 licensed, so Telemessage should be fine with this ;)
Read more at @micahflee blog post at https://micahflee.com/heres-the-source-code-for-the-unofficial-signal-app-used-by-trump-officials/