The article is not about free sites, but about free services being integrated to those sites.
For example, integrating free version of disqus for comments adds trackers to the site that even the owner is unaware of. Another is Oracle’s AddThis which integrates social media sharing options but sends data to Oracle even if nothing is shared. Similarly integrating payment services can add trackers.
What is worrying is that even the website creator might not know about these trackers and hence, cannot disclose this to the users.
Yes. this is well established, but how many people actually know about this or actually understand what this means?