Slack did not file a complaint because they're losing. This is what it says in the article: Slack filed a complaint with the EU in 2020, claiming that Microsoft was "force installing [Teams] for millions, blocking its removal, and hiding the true cost to enterprise customers." And this is investigated 3 years later than the complaint. Microsoft has a history of installing bloatware and changing default clients to open with their own software.
They're not collateral damage of the strikers, they're collateral of the same executives that dragged their assets thinking they would win.