Microsoft's strategy of Embrace and Extend is well-documented in the archive of this blog. It's how a second or third entrant in a market gains access to the users of the earlier products (embrace) and how, once they've become the leader, they take users with them without sharing them with competitors (extend). This approach really worked for them in the 80s and 90s. But then they lost their hunger, and the tables got turned on them in subsequent decades. 

So now they've shipped a product clearly aimed at Slack, which has become a juggernaut largely by being a backplane for bot-like apps. Slack, as we explored here in the summer of 2015, got there by having a wonderfully open and web-like API. They are to be commended for this.

So the big question for Microsoft and for the future of the market is this: Do they interop with Slack? Can I use the Microsoft client without forcing my teammates to adopt it? Can they continue to use Slack while I use Microsoft? If so, they will certainly gain a user base. 

And, if they interop, then Microsoft is returning to a playbook that worked for them once, and (my belief) probably would work again.

A lower level of interop would be the ability to run Slack-compatible apps. That would mean they wouldn't have to be rewritten to work in Microsoft's new environment. That means more utility for users. But it's more work for Microsoft. 

BTW, the Microsoft of the 2000s would have considered this a silly question. We're Microsoft, people interop with us, not vice versa. That didn't work very well. Humility is a good trait, even for market-dominant giants.