No, they're arguing that since Microsoft has a monopoly on office productivity software[0], adding new features to it that are in direction competition with other products is anti-competitive behaviour.
And they are right about that. Microsoft has been repeatedly convicted for this behaviour (Windows Desktop -> Internet Explorer, and Windows Desktop -> Windows Media Player). Trying to argue otherwise would, IMNSHO, be silly. The only valid question remains, does Slack's offering itself (corporate IM) fall under the office productivity umbrella or not?
[0] and no, you do not have to own 100% of the market before being considered a monopoly
And they are right about that. Microsoft has been repeatedly convicted for this behaviour (Windows Desktop -> Internet Explorer, and Windows Desktop -> Windows Media Player). Trying to argue otherwise would, IMNSHO, be silly. The only valid question remains, does Slack's offering itself (corporate IM) fall under the office productivity umbrella or not?
[0] and no, you do not have to own 100% of the market before being considered a monopoly