It's unclear if these were private messages. It's possible these were posted on public channels. AFAIK, Slack doesn't allow companies/IT access to private messages unless there are legal implications.
https://www.businessinsider.com/netflix-fires-marketing-exec...
> Netflix has fired three senior executives within its film marketing team for airing complaints via private Slack messages
Yes, slack absolutely allows IT to view your private messages.
https://www.vox.com/recode/2020/1/24/21079275/slack-private-...