The group is unencrypted because it's not possible to encrypt group chats in Telegram, because Telegram is that bad.

They're probably assuming the chats aren't encrypted because telegram doesn't encrypt those. Source

Multi-device End-to-end encrypted chats are a mess

The concept of End-to-End Encryption has no limits for the number of communicating devices. However, if you want to access your end-to-end encrypted chats from multiple devices, you’re facing many technical difficulties, especially when it comes to connecting new devices, loading chat history and restoring backups.

Most of our competitors (notably, Whatsapp and iMessage) solve these problems in ways that make their end-to-end encryption useless (this is a big topic, so requires a separate manual [poster note, that link goes to a 'manual' that hasn't been filled out lol]). To solve them in a secure way, you’d have to sacrifice usability and some of the features you’re used to – the result would never be as fluent and simple as what we offer in Cloud Chats.

Telegram says they don't encrypt them and tries to imply that people who actually know how to use cryptography failed to solve this problem because they couldn't solve it with their shitty self rolled 'encryption' algo that hasn't been peer reviewed (unlike the signal protocol)