It's not anticompetitive if the actual product is easy/cheap to implement (especially on Microsoft's existing infrastructure). If they can do something much cheaper than Slack can, and they can turn a profit at a much lower price point. The Slack product has a lot of delivered value for teams, but that doesn't mean that they have a right to high margins or high costs.
You're not required to use Teams on Office365. The anti-competitive behavior would be if you couldn't turn it off, and it had to be on due to core functionality with Office. Most people just use o365 for exchange and office apps.
Well, now that slack has so "genuinely" welcomed microsoft as a competitor, I don't think they can turn around and argue the opposite for the same offering.
Slack is going to look dumb if it ever sues Microsoft.
"But, your honor, Slack told the world they welcome competition from us in this infamous letter from November 2016!"
I get your point, but, if an anti-trust suit ever happened, it would be DoJ that sues MS, not Slack.
Microsoft has nothing close to a monopoly in this space, so no.
In office applications? That is the one they would be abusing.