The site isolation claims for the past 3 years are made by madaidan, and not u/subzer0carnage on reddit. To debunk your very own claim, let me cite the developer of Mull and DivestOS himself, from reddit, from 3 weeks ago.

http://web.archive.org/web/20220322130653/https://old.reddit.com/r/PrivacyGuides/comments/t2f4mz/question_about_firefox_android_site_isolation/hylvi0o?context=3

“Site Isolation” on its own is nonsense that people need to stop parroting. When it comes to browsers you have per-site PROCESS and DATA isolation. On desktop:

• Chromium by default has per-site process isolation
• Chromium by default has no per-site data isolation
• Firefox by default has per-site process isolation
• Firefox by default has some per-site data isolation
  • In private browsing mode has strong per-site data isolation On Android:
• Chromium by default has per-site process isolation
• Chromium by default has no per-site data isolation
  • Bromite and Mulch enables strong per-site data isolation
• Firefox has no per-site process isolation or Android isolateProcess usage
  • This is the big issue people usually mean
• Firefox by default has little per-site data isolation
  • Mull enables strong per-site data isolation Useful sites:
• https://divestos.org/index.php?page=browsers
• https://privacytests.org/ Also because it isn’t obvious, Mull and Mulch are my projects.