While ad blockers do stop a lot of third party ads and iframes, and probably some tracking, I don't think that ad blockers are stopping most tracking by web sites. Not the kind this article is talking about, anyway. It's quite simple for sites to track user activity in ways that ad blockers can't easily stop. And I suspect it's becoming more likely quickly, purely on the anecdotal number of sites I visit that now detect ad blockers.
Some of this article seems like FUD though. It's pretty hard to think of what keystrokes you would type into a web site that you don't want to send the web site. The main thing they're getting by tracking keystrokes that they didn't get before is typos and form activity that you start and then decide to clear and bail out. I'd speculate that this is being used and is most important for sales conversion pages, where they monitor your activity to try to figure out if you started out interested in buying something but then decided not to buy.
Web sites cannot track your keystrokes when you're using a different site / tab in your browser, they can't track keystrokes when you use another program besides the browser, they can only track when you're interacting with the site. So what sensitive data can they get that you weren't already giving them?