Firefox 85 partitions all of the following caches by the top-level site being visited

This means that while the identification number is not the same cross-website (webs with different top-level domain will get different value), it will still work to identify the user accessing that website in a way that “can be stored almost persistently and cannot be easily cleared by the user […] by flushing the cache, closing the browser or restarting the operating system, using a VPN or installing AdBlockers”. Which was what this tracking method was claiming to do.

Going to https://demo.supercookie.me/ still gives you a consistent ID on Firefox (and I would guess, Chrome too), across reboots that isn’t protected by adblocker or similar privacy tool. Though one thing they are wrong about is incognito mode, at least in Firefox, the cache for incognito mode is new every session so the id would change if you do use incognito.