Replace "are using Slack" with "discussing at the bar across the street from the office" and what changes about the situation? If the company has an obligation to look into something, they have to look into it. They don't necessarily (and in my opinion shouldn't ever) have the need to, say, record audio of everything you ever do. What does the gray-er area of conversation in the break room at the office look like? What about the darker, gray-er area of conversation in the parking lot before you drive home at the end of the day?
My personal opinion, having avoided the MS IM client at work, is that you never say anything in writing that you wouldn't walk into the CEOs office and say to him in person. Chat of any kind, Slack included, is "in writing" and will have the same full force legal effect as email, so who's honestly surprised by this news?
This idea that, because you can't perfectly stop something, you shouldn't try to do it at all is madness. Yes, they could get around it. But in this situation, they're not.
My personal opinion, having avoided the MS IM client at work, is that you never say anything in writing that you wouldn't walk into the CEOs office and say to him in person. Chat of any kind, Slack included, is "in writing" and will have the same full force legal effect as email, so who's honestly surprised by this news?