If you've ever worked for a large American company, you know that nothing you do on company equipment or with a service the company pays for is ever private, and you should never assume it is.
I'll be honest, I always thought Slack DMs were viewable by the admin. As a Slack admin myself, I always assumed I had that ability. Never used so, I never found out I was wrong, but just always assumed it was there.
To me this is a no-op: Anyone who worked for a large American company should have assumed that this ability was always there or could be there in the future, or at the very least, your employer could have always required you to log in and show them your DMs.