You're right, the article seems to skim over this fact.
But the same is true for Teams. Teams is bankrolled by Microsoft. Ok, it's fair from Microsoft's point of view - if they can afford, it makes sense.
But from a customer perspective you have a subpar product that's pushed onto you through sheer cost perspective: it's technically free per your other Microsoft products so nevermind it's awful (of course, speaking about end-users, not decision makers on top of end users).
So, both Slack and Teams are technically black holes financially, but one prevails simply because it's a product of a company that had decades of "being there first".
And that's the gist of anti-trust laws. Simply being there first and amassing wealth doesn't give you the right to use that wealth to stifle competition and push your product in a market that's supposed to be competitive.