I went to a sushi restaurant that was owned and operated by an all white staff. Everything on the menu was ordinary Japanese food like you'd find at any Japanese restaurant. The only exception to this was the cheesecake gyoza that I had for dessert.
I was wondering: "Is this a Japanese dish?" It was delicious and well prepared. It was made by someone who clearly knew a lot about Japanese food and their restaurant was about as high quality as any actual Japanese place. Still, I would have a very hard time calling this gyoza a Japanese dish. This isn't a word game either. I 100% consider the sushi that I ate at the restaurant to be Japanese food. It was normal sushi. I'm not just doing a bit where I call anything cooked by a non-Japanese person to be non-Japanese.
The question of whether a dish is Japanese doesn't come into play when it's prepared; the question comes into play when the dish is imagined and no amount of faithfulness to the original people can affect the outcome. For all I know, the Japanese could, over the next 1,000 years, innovate and change their cuisine so much that it's barely recognizable and has no real resemblance to what it is now and that food would be more Japanese than a faithfully executed minor innovation from a non-Japanese person. There's something over and above a list of ingredients and preparation methods that makes a food "Japanese".
What that thing seems to be, is just a "divine" right bestowed upon the Japanese to create Japanese food. Their culture is ultimately Their culture and that means that it's theirs to innovate, to change, to add too, or to eliminate parts of. There's an inherent legitimacy to the Japanese coming up with something radically different and calling it Japanese and that inherent legitimacy makes it so that nothing they ever do has to seek approval. They are free to express themselves in their kitchen and whether a dish is successful or not, it's their dish.
While one race can imitate the cultural artifacts of another, such as preparing their ethnic dishes, there is an inherent limitation to how much they can authentically innovate it before it stops being part of that culture all together. A civic nationalist cook would say something like "Anyone who mixes white rice, seaweed, and raw fish has something to add to Japanese culture." But the Japanese people might not agree.