Food is constantly evolving.
Something that is what it claims to be; “authentic” isn’t a free-standing adjective for cuisine (I mean, beyond that if it is food that it is intentionally prepared, it is “authentic cuisine”.) It applies only to some claim about the cuisine, e.g., that is of a particular origin. So, the question isn’t “is X dish authentic” but, e.g., “is X dish authentically Mexican”
> I ate “authentic” Alsatian food that would be entirely unheard of a thousand years ago in the region
That’s…not surprising. I mean, its not “authentic pre-1000-BCE Alsatian food”, then, but…
(OTOH, American-style tacos are an invention of Americans in the United States. They are authentic American cuisine, but not authentic Mexican cuisine. They are inspired by Mexican cuisine, and they have also inspired some things in newer Mexican and Mexican-American cuisine, but they themselves are just plain American.)
Tomatoes came to Europe from America as a consequence of settlement, so you have that backwards.
> That makes tomatoes in Italy as authentic as Mexican cuisine.
Well, it makes it almost as old as Spanish-derived Mexican identity. There is quite a bit of cuisine now identified as “Mexican” that is older, e.g., of Aztec origin.
i dont dispute that authenticity can be hard to nail down at times but there are plenty of situations where it isn't, so why does the whole concept need to be tossed out? saying american tacos are not like mexican tacos is objectively true; if someone lets that affect how they enjoy american tacos, that's a personal problem, but the statement itself doesnt imply american tacos have no value
i feel like this is a touchier subject than people realize. many cultures have rich and storied culinary traditions regardless of all the one-off examples like fry-bread and so on. saying "authenticity is meaningless" is easily interpreted as an attack even if that wasn't the intent.
Since "authentic" is such a weird and ephemeral thing that's hard to pin down, policing people's language generally just comes across as petty and small.