If you have one browsing history and one Twitter profile, it is easy to compare them. However, the main technical challenge is that we have to find some way to compare a given browsing history to every single profile on Twitter, which in theory is hundreds of millions of comparisons, plus we would have to find some way to get hundreds of millions of news feeds. Solving this took us several months. Of course in the paper we explained how we reduced the search space so we only have to consider a limited set of candidates, and only have to crawl part of the news feed in real time.
Of course if a couple of grad students could do this in a few months with only publicly available data (plus donated browsing history), then I'm sure ad networks could easily do this too, and given how many ad networks have parts of your browsing history I would say that it's scary that it's so "easy."
This is useful in that it limits the set of people who have access to your browsing history. But first party tracking will still be a thing (i.e. if you are visiting a website, the owners of that website will know you are visiting it). Actually there is a paper that says that if you only use the New York Times links that people click on from Twitter, it still makes you uniquely identifiable in many cases, so a company like the NYT could (theoretically) track people without using any third party tracking at all.
