They say that their data comes from "Partnerships":

"Rich data pre-analyzed by global partners like DPSs, ISPs, measurement companies and corporate intelligence firms"

What is a "corporate intelligence firm"? What is a "DPS"? How can ISP's provide the data?

And from "Website & Apps Owners":

Data directly measured through first party analytics (e.g. Google Analytics) of millions of websites and apps.

But how? If they don't have access to a GA account, how do they know? Is if the website uses some third party asset (js, css, etc..)?