Telegrams crypto is based on their own, contested protocol and is disabled by default. Telling someone to use that for secure communications is difficult because you also need to remind people to turn on encrypted communications.

Furthermore, signal and WhatsApp do e2e in group chats where telegram doesn't.

Dont get me wrong, I use Telegram daily (it's desktop clients far outperform any of its competitors), but it's not as secure as WhatsApp or Signal.

I'd classify Telegram as "maybe secure" but I wouldn't recommend it to people depending on the security of their messenger application.