Intellectual work is the work of abstraction, recombination and imagination. Conversely, when I'm an activist, I work for a particular group to particular ends. I've written all sorts of literature for them, participated in various organizing efforts, run meetings, and helped train other organizers. So my activist work is based in allegiance to a specific organization to which I'm quite monogamous. I'll show up to other peoples' rallies and actions, but my activist time is concentrated in one place.
Activist work is much more clearly ends-oriented and based in immediate and tangible results.
So while activist undertakings refer back to a specific organization or project, intellectual undertakings many times do not. Instead, as in this essay, the left is 'left' unspecified. Strictly speaking, there is no such thing as the American left. As intellectuals, we're 'left' with the uneasy task of specifying our own positions without recourse to a specific party or organization. Instead, terms like Marxist or feminist are generally emptied of any specific meaning and hitched up to adjectives like rail cars.