The method offered at the post urges you to install the desktop Kindle software, and de-DRMs files from its storage. That hasn't always worked in the last year, as Amazon tried to obfuscate the key storage in the desktop versions, and there's been a kind of race between Amazon and the authors of the de-DRMing tools.

I don't know what the status is now, but I want to emphasize that if you own a physical Kindle you don't need to install the Kindle Windows/Mac reader at all. Just follow the instructions on the blogpost linked to in the Wired article and customize the plugin with the Kindle's serial number. To de-DRM, you then connect the Kindle to your desktop and just Add the books off Kindle as a disk drive into the Calibre library.