1. Telegram's E2EE is not on by default, therefore 99% of users don't use it.
2. Telegram's E2EE is not advertising authentication, therefore ~90% of the people using it don't check for MITM attacks, therefore majority of E2EE is useless against active attackers.
3. Telegram's E2EE does not work across devices, therefore majority people who use secret chats also use non-secret chats because desktop client don't support it.
4. 100% of Telegram's group conversations can be eavesdropped by the server, because Telegram doesn't have E2EE for group chats.
Complaining about possible cribs in how Telegram implemented the protocol from cryptographic primitives is an insignificant problem compared to the fact the entire protocol is fundamentally FUBAR, how it's so glaringly obvious you can't even fill out a CVE form.
If Signal had vulnerability where 100% of group conversations were not properly end-to-end encrypted, every newspaper in the world would publish something about it. However, with Telegram it has been spun as a "feature".
Another big problem is Telegram has been mentioned by hundreds of publications as "Secure apps like Signal, WhatsApp and Telegram".
To experts it's like hearing news spout "Great writers like Leo Tolstoy, Paulo Coelho, and Stephanie Meyer", or "Great bunker materials like reinforced concrete, steel, and MDF".
Repeatedly claimed, anyone would make mental associations between the three, but when you actually find out what they're about you can't believe your ears.