This article incorrectly implies that Telegram is end-to-end encrypted, by putting it in the same line as WhatsApp and Signal. Telegram doesn't even try to be end-to-end-encrypted by default. WhatsApp claims to be end-to-end-encrypted, but it's not open-source, Signal is end-to-end-encrypted.
And explicitly does not encrypt metadata.
Meanwhile NSA top brass publicly stated, "We kill people based on metadata."