(Here I'm focusing on part 2 and the conclusion of the article, not part 1.)
This article suggests that RMS ought to make his own leftist beliefs an explicit part of the free software movement. As the author is aware, RMS has generally tried not to do this, recognizing that free software advocates can have a wide range of beliefs about other issues, even if a plurality of people in the movement are hold leftist views. In general, RMS has stood for the effort of trying to find common cause around specific issues and principles among people who might radically disagree about other things (another example was the League for Programming Freedom, a single-issue campaign against software patents that actively aspired to include people who disagree with RMS about free software!).
It's true that the big-tent concept could fail as a tactical matter if it turns out that the success of one cause really is indispensable to the success of another, which is a big part of what this article is claiming. But I wish the article had engaged a bit more with the fact that RMS does not appear to agree with this analysis and has consistently wanted people with non-leftist views to join the free software movement, including enthusiastic supporters of capitalism and private property. In other words, it's not an oversight or an accident. (To be clear about this, the classic formulation of RMS's free software philosophy calls for not regarding software as private property -- for example, in "Why Software Should Not Have Owners" -- but stakes out no position on regarding, say, financial instruments, airplanes, factories, companies, plots of land, or apartment buildings as private property. Hence many more people could agree with it.)