They've got two scores for Telegram on their score card[0]: "Telegram" scores poorly (4 out of 7), "Telegram (secret chats)" scores perfectly (7 out of 7). The quality of the encryption algorithm itself doesn't factor into it, though Telegram gets a check for having a recent audit.

Even if the crypto was good, the cognitive load of having to decide whether you want a chat to be secret or not makes it a bad choice IMO, especially if it comes with a downside.

[0] https://www.eff.org/secure-messaging-scorecard