First, the study specifically avoided self identification and labeling. They clustered people based on their answers to core belief questions. From which they found 7 key clusters. One of those the study labeled Progressive Activists. But that's a label put on after the fact. So it doesn't mean 8% of the population identify as progressive activist. Just that from the study, there's 8% of the population with very similar core beliefs.
Then the study demonstrates that self labels like being left, right, middle, conservative, Democrat, Republican, etc, are bad predictors of shared common issues.
Only 3 out of the 7 groups cared most about jobs, and only one group, the smallest one, cared most about immigration.
The groups the study labeled liberals make up 26%, and the ones labeled conservative 25%. But again, that's the labels the study gave people based on their answers.
Also, the wealthiest and withest cluster identified of them all is the Devoted Conservative.
Finally, the whole point of the study is that appart from the wings of progressist activist (composed of young professionals mostly), and from the traditional (old, white middle class mostly) and devoted conservatives (old, white and wealthiest mostly), all other groups are much more willing to compromise and have varying beliefs depending on the situation, and are most bothered by the wings polarizing the country. Thus when you look at the silent majority, they are much more diversified in opinions.