The reason the server can track who is hovering over links is the browser apparently requests the background image only when the interaction happens. I certainly didn't expect that!
This seems to be an example of browser optimization leaking private information. There's an obvious way to fix that: deoptimize. Instead of lazily downloading resources as they are needed, request all the linked resources when the page loads, and do it only once. Now they can't tell whether the requests were due to user interaction or just normal browser behavior.
I wonder if uBlock Origin can deal with this. I know it's got a lot of options for blocking weird information leaks like this, webfonts being an example. Firefox also has fingerprinting resistance, I wonder if it resists this.