Federation and security is a hard problem. The code is there for anyone to use, but it's an entirely reasonable and prudent security decision to announce your lack of support for a forked version using the same servers.
(Also, does Telegram have a version without Google Play Services?)
The XMPP ecosystem has federation, security and almost every client can talk to almost every server. If Moxie Marlinspike claims that interoperability does not work well in the context of instant messaging, I see this as him trying to manipulate the users so they do not judge him for his behaviour.
The XMPP ecosystem doesn't have security anywhere comparable to what we're talking about (well-implemented, end-to-end, forward-secure, multi-device messaging).
(Also, does Telegram have a version without Google Play Services?)