Telegram only uses end to end encryption for secret chats. All other chats are only encrypted on the wire with Telegram's keys. Your comment was encrypted on the wire to HN but that's not going to do anything to keep it away from the FBI. The majority of all Telegram messages are only secured by Telegram's unwillingness to cave to outside pressure. It's in plaintext as far as they're concerned.
also curious - how does telegram support encryption for chatrooms without the parties being known in advance? or are those chats not encrypted?