But so does the author's solution of switching to Fastmail so surely that's not a non-negotiable feature. Manual labeling and filtering still works.

I've seen those categorizations used, and to me they're totally useless. I instead filter messages in the Filter settings, and apply labels and other things as I wish. This works exactly as I need it, unlike the Google categorizations.

Disabling those Primary/Promotions/etc. tabs is the first thing I do whenever I setup a new Gmail account.

I just want to see my inbox, I don't trust an AI filter to sort emails for me.