“The tracking method works even in the browser’s incognito mode and is not cleared by flushing the cache, closing the browser or restarting the operating system, using a VPN or installing AdBlockers.”
Browsers keep a separate favicon cache, and it survives everything you do to clear your tracks. A site can write an ID into it with a few redirects and read it back later. Incognito doesn’t touch it. Every convenience feature browsers bolt on turns into another place to store a tracker.