Channels are extremely public, in the sense that you can access them even without installing telegram [1] and there are websites which can generate RSS feeds for all the posts of a channel or things like that [2]. So this is not exactly a private conversation we're talking about, more like the printing of broadsheets to be nailed to every other house's wall.

[1] For example, this is a post in Telegram's own public channel https://t.me/telegram/167 [2] For example, this two: https://tg.i-c-a.su/ and https://rss.app/rss-feed/create-Telegram-rss-feed