They could release a version of the server codebase that is not the production one, and who knows? They could put on the Google Play Store/Apple App Store a version of the client compiled from different sources (and mobile builds are not reproducible), and who knows?
Also the main benefit of FOSS is the possibility to take the source code, modify it, implement new functionality, fork it, and so on. And Signal is against that. So to me it's like a proprietary software.
Telegram on the other hand it doesn't provide an open source server, but the clients are 100% open, you can compile a client from source, modify a client, make third party clients that implements the Telegram protocol with the official libraries. All of that it's useful not only for improving existing clients, but for automating stuff, there are libraries for basically every programming language to interact with Telegram. That to me means being open source, and what distinguish Telegram from all the other applications.