They're looking at how users use Google Search because the data's there. They're making a competitor to Google Search. That doesn't mean they're rebuilding Google Search's SERPs, or making a Google Search “clone”; I've got results from Cliqz for queries I'm confident have never been put into Google before, meaning it's functioning as an independent search engine.
This. Having worked in past life for one of their competitors, can confirm - what users click on (in SERP) is one of the most powerful signals for ranking. And who got (almost) all the clicks in the world? Google!
That's why it's so damn hard to beat them. It's the unreasonable effectiveness of data: more data (which they have almost all of) usually beats a smarter algorithm, and with 20 years R&D, theirs is surely not dumb.
Do the clicks belong to users or Google, that's an interesting question, though.