What was pizza when it was invented? it was the fast food of the Italian proletariat. Most of the "true Italian pizza" you can find nowadays is going to be from an expensive bourgeois restaurant, or rarely from an actual Italian family place if you're lucky to live near one. "Authentic pizza" is mostly not consumed by members of the working class, so how it could it actually be a real example of that culture?
Let's say I lived in a town with no Serbian population. I start a restaurant advertising it serving Authentic Serbian Cuisine. By the argument you've lined out here, what I served in this restaurant is irrelevant, since it's the only restaurant claiming to be serving Serbian food.
Let me make my point here instead of continuing the snark: You cannot keep appealing to a notion of authenticity while also rejecting any kind of cultural ties that the word implies. I would argue that authenticity in food comes from things like ingredients, taste, preparation, and even the way it's served and eaten. The argument that you're supporting, that food authenticity arrives not from the food itself, but from price point and class of people consuming it, is a bit silly.
The word "authentic" implies some kind of tie to tradition. I couldn't give a fuck about tradition, but to try to divorce the word "authentic" from any kind of traditional framing is Quixotic.