I'm not seeing how you could draw that conclusion. The more likely explanation is that they are telling people not to build apps around it (and I assume thus the apis aren't well designed for adoption by other apps).
> This repository is used by the Signal client apps (Android, iOS, and Desktop)
as well as server-side. Use outside of Signal is unsupported.
> This repository is used by the Signal client apps (Android, iOS, and Desktop) as well as server-side. Use outside of Signal is unsupported.
https://github.com/signalapp/libsignal