Having just seen a video on making
budae-jjigae -- the authentic and popular Korean stew that includes Spam and canned beans (perhaps not too surprisingly,
budae jjigae translates "army base stew") -- I thought I'd search and see if this dish had ever been mentinoed here -- and so it has. (And if you're interested, here's a link to the video:
https://www.youtube.com/watch?v=euVyBKNfxkk ) This is a topic that has long made me smile -- and squirm -- not just because of new vs. old, but also because of "what's on hand" and "that's how mom made it." A lot of the world's cuisine was just what was available (from pasta carbonara to chicken Marengo -- and, I suspect, just about everything else ever cooked, at least the first time it was prepared). As for what mom made, which is authentic tuna salad -- mayo and onions with albacore or Miracle Whip and pickle relish with chunk light.. Personal preference (and family history) is the albacore with mayo, but I don't think that is more authentic than the alternative.
As I've traveled, I've had food all over the world that was undeniably authentic and yet nothing like someone else made it. For example, fish amok in Cambodia. Some versions are soupy and served in a coconut. Others are more like a firm custard and wrapped in a banana leaf. Both versions were eaten in Cambodia, prepared by Cambodians, so both were authentic. The older a recipe is, the more versions there are likely to be. I think the idea of having an "absolute" version is a relatively new idea -- just like the people who think current borders have always existed.
I think the general discussion reflects much of this thinking, so I'm probably preaching to the choir.
The more one researches, the more one finds that "authentic" is elusive, simply because so many versions of things can be authentic, depending on the timing. If one thinks in terms of absolutes, then a Mongolian's roasted mutton and fermented mares' milk might be among the only truly authentic dishes left, as there are no variations on the theme, no new spins, and no introduced ingredients. That said, I think, as toria notes above, a wide range of options can be accepted if a dish is faithful to a place it originated, irrespective of what history has brought into play. For example, Chinese American food is authentic to the Chinese in America who created it, to try to hang on to memories of home.
I think the danger lies in ruling something out as inauthentic just because it is not what one has experienced previously. That's when one needs to move into "not my preference" mode. And then there is the question of "is it tasty?" I have discovered in many places that the authentic version of something isn't nearly as tasty as a slightly more tarted up version of the dish. So I think it's worth remembering that authentic doesn't always mean good (Icelandic rotted shark comes to mind).
I actually try to avoid the word "authentic" when writing about food, or else I'll say, "this is authentic, but it is only one version among many." Or I'll say "historic" or "this is how this person in the past made it." Just because "authentic" is usually not as definitive as I might wish.