The problem is not just an AGPL violation here, even though the license explicitly requires to show the code if you are providing a service on top of it. According to AGPL-3, if you are using the service you are the user. Good luck anyway submitting such a request to them at this point.
The actual problem is that #signal is no longer willing to publicly share the sources of their server platform, which is what #signalapp users criticized the most about others in the past, #telegram in particular.
No, the license requires that you share the code to your users that request it, not that the code is made public to everyone. Has anybody formaly requested the code to them?
I mean really presuring them to release it by threatening legal actions?