Just because everybody does it, doesn't make it right.
It can be occupy protesters who own iPhones to coordinate or open source purists who use Slack to join projects they love or environmentalists filling their gas tank. In the end, there has to be a more practical strategy that enables a person to fight for the world they want while living in the world that exists.
That is inexcusable, though.
There are more alternatives than you can count, if you use Matrix, Mattermost, some webirc system with included bouncer, or whatever.
edit- In this case, the open source purist has to join Slack if only to beg people to go to IRC, a move that is at that point more energy than the community will think worth the purists principled gain.
I don't share his definition of freedom applied to software, simple as that. I also don't think most of the society shares his values, i often get the idea that he lives in a bubble. Like somebody preaching abstinence instead of birth-control.
I get that "free software" has advantages, but for me it also has disadvantages and therefore can and should not replace all the "unfree software".