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.
Doesn't the fact that nobody has built apps on it indicate the license (AGPL 3) is a real issue for its ecosystem?