So if you get a 3rd party cookie from www.marketingsite.com, but never visited that site, it will be deleted in 24 hours. But if you get a 3rd party cookie from facebook, and you're a semi-active facebook user, the cookie will be kept.
The article mentions you get redirected to a website before going to your destination, so the third-party cookie is no longer 'third' but 'first' since you (unbeknownst to you) visited the site.
In the security blog post, they explicitly say "An origin will be cleared if it [...] is classified as a tracker in our Tracking Protection list" and "[has] No origin with the same base domain (eTLD+1) has a user-interaction permission".
So it's not a catch-all for any and all redirect trackers, just those Mozilla knows about, and whitelists trackers that are first-party in spirit. I do wish they didn't frame it as "we're blocking redirect trackers" but instead as "we extended our tracking blacklist to include known redirect trackers, unless we have a good reason to think you allow them".