Really surprised that a CEO would make a comment like that on the record.
Well it’s not anti-competitive to be competitive...
"There’s fewer choices for people," is such a short quote that I would avoid judging it on its face without seeing the full context.
Slack's competitors include Google, Microsoft, and Facebook. You can say that all of those companies have terrible products if you want (Hangouts, Teams, and FB for Work), but all have been positioned to act as Slack alternatives.
I don't think Slack is worried about looking anti-competitive in that crowd.
"Competition is for losers"..."Always Aim for a Monopoly" -- Peter Thiel