This story is just a summary of: https://www.theverge.com/2019/4/9/18301736/slack-microsoft-o...

It also contains less information as it makes it seem like Slack and O365 are collaborating when they are not.

tl;dr: "Microsoft has shown in the past that it’s willing to partner with rivals like Dropbox, but Slack’s Office 365 integration was created using public APIs and with no special help from Microsoft. This Slack integration comes just months after Microsoft launched its free version of Teams to rival Slack, just weeks after the company unveiled creative new conference call features for its Slack competitor." (from the Verge article)