[em3rgent0rdr]

If the compromise of the secret servers causes the entire network to go down, then that secret is a weakest link, so it's not "security plus obscurity".

I don't know much about Telegram's system, but if it is run on centralized servers, then that is a serious weakness...adversaries who have the capability to find and disrupt can exploit that so even though the communication may be end-to-end secure, the entire system however is not resilant against such attacks.

[kelnos]

I think it's safe to assume that obscurity is not the only security feature protecting the secret servers. A good defense-in-depth strategy might include some amount of obscurity, along with other measures.