Arkenfox does far more than that, just look at the user.js. Among others, it spoofs the time zone, number of cores, window size and many other attributes that assist fingerprinting. It basically mimics the settings of the Tor browser.
privacy.resistFingerprinting = true is basically activating most of the Tor browser features in baseline Firefox. That's why it is turned off by default. It does all the things you listed above only at a lower level than a user script. It's been in Firefox for over a decade.
The flag was in fact designed to control the activation of the Tor browser uplift features, and reduce maintenance issues. That way the Tor browser could pretty much just be Firefox with certain flags turned on.
