Yep, you're 100% right.
I was genuinely shocked to learn that slack admins can configure it so that certain folks can read private chats. Once that became known, most of my team offloaded to discord/keybase (at a previous gig with toxic leadership).
the only reason companies use slack/teams/gmail is because those services understand the data belongs to their clients (the employer) and will provide ways to access private chats/change ownership of private documents/etc with zero trouble.