Looks like we're both not entirely correct (and probably both skipped the security blog posts!). In the security blog post, they explicitly say "An origin will

Looks like we're both not entirely correct (and probably both skipped the security blog posts!). 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".