The problem is open source?
The author suffers from some convoluted misunderstanding of open source/free software that is hard to untangle. Is he saying that Slack is open source? It isn't. Is he saying FOSS developers are cheapskates? That's hard to quantify, some teams of two really just can't afford to pay for any services through no fault of their own, some others spend their own money to provide services for thousands or millions of members of the community (Diaspora, Gitlab, etc.). Is he saying Slack is mostly used by FOSS developers? Where are those numbers to back that up? Or is he saying "having a free version of a service is like being open source"? In that case, reading and understanding an introduction to FOSS might be in order.