Their client code is not usable because they don't provide working instructions for compiling it. If you can't compile it, they can't have reproducible builds.
For example, see this issue from 2019 which the developers still haven't replied to:
https://github.com/TelegramMessenger/Telegram-iOS/issues/97
There are a lot more issues about people not being able to use the code, none of which get any replies from the owners.
(uses this script: https://github.com/TelegramMessenger/Telegram-iOS/blob/maste...)
And here are few recent examples of them helping people to solve build issues: https://github.com/TelegramMessenger/Telegram-iOS/issues/968... https://github.com/TelegramMessenger/Telegram-iOS/issues/986... https://github.com/TelegramMessenger/Telegram-iOS/issues/992...
How were you digging to find an irrelevant issue yet omitted all of this? I really don't see the picture you're trying to paint.
Incidentally, Signal is still not available on F-Droid, and Signal developers are known to be hostile towards third-party clients (for "security reasons." Almost like I've heard that argument before from a certain smartphone manufacturer...)