They are doing everything they can to discourage third party app development.
I’d say you’re moving the goalpost. Other than the hostility the founder showed towards LibreSignal nearly 10 years ago now, can you source any evidence to support your claim?
Lots of red flags here in Github: https://github.com/signalapp/Signal-Android/issues/9044
That link, and I could be missing it, has nothing to do with what I claimed. Mind editing your post and quoting a red flag linked at the source you provided?
Some of my favourite red flags:
Signal’s dependence on Google libraries: https://github.com/signalapp/Signal-Android/issues/9044#issuecomment-535194837
Signal dev bullshitting a non-answer and then hilariously refuting his non-answer: https://github.com/signalapp/Signal-Android/issues/9044#issuecomment-534340623
Signal hiding its serverside source code for many months: https://github.com/signalapp/Signal-Android/issues/11101
You can find many more examples.
The last one about server side code, together with Signal’s funding sources and their obsession with phone numbers code leads me to suspect that Signal is just a honeypot by US intelligence.