> So in this case, as an American browsing high-end groceries, I figured "Dijon Moutarde" would be an "appellation d'origine contrôlée" but I am mildly surprised to find out that it is not, and that I should look for "Moutarde de Bourgogne" instead.

I'm French and I can attest that my friends didn't know that [AOC for mustard is "Moutarde de Bourgogne" not "Dijon"] before I learned it a few years ago (well into adulthood) and told them about it. I think it's one of the few cases like that where the AOC doesn't match the intuition.