It's not about what build they are running. It matters because somebody just glancing at it might misinterpret the situation as "Telegram is open source", but it actually isn't because the server isn't. Just some clients are, which is pretty useless if you can't run a server to talk to them. Just for arguments sake, let's say Telegram gets busted tomorrow in an international sting operation and all their servers get taken offline. The clients will be entirely useless at that point, somebody would have to reverse engineer the server.