The "small tribe of devoted conservatives" is much, much larger than "progressive activists". Activists in general tend to be richer and better educated and since income corresponds with race, white. That's because if you're poor, you don't have the time or the money to become an activist.
This isn't really saying very much.
[1] https://static1.squarespace.com/static/5a70a7c3010027736a227...
That said, the actual amount of racially homogeneity described in the study is:
Progressive Activists: - Eleven percent more likely to be white - 80% V. 69% – Seven percent more likely to be between ages 18 and 29 - 28% V. 21% – Twice as likely to have completed college - 59% V. 29%
Devoted Conservatives: - Nineteen percent more likely to be white - 88% V. 69% – Fourteen percent more likely to be older than 65 - 34% V. 20% - and much less likely to be born between 1985 and 2000 - 11% V. 27% – More likely to come from the South - 45% V. 38%
Link to study in question: https://static1.squarespace.com/static/5a70a7c3010027736a227...
Regardless the study sorted people into groups and then the article attempts to draw conclusions from that sorting that doesn't necessarily follow.
The demographics of the "progressive activists" doesn't say anything about the makeup of people are likely to support policies that could be defined as politically correct--just the makeup of people who are likely to support all progressive policies.
The only thing you can get from the GP's quote is that white people tend to be overrepresented on both extremes of the political spectrum.