Do you have anything to back up your claims or is it just that you'd prefer China not to be involved? Because I've found multiple sources backing up the claim that China was in fact involved.

This book (https://books.google.com/books?id=iyQps4i41JoC&pg=PA344&redi...) mentions Southern China as one of the origins of narezushi.

This one (https://www.businessinsider.com/the-complete-history-of-sush...) goes even further by specifically talking of Southern China as the origin and suggests that the influence of northern invaders explains why you won't find much signs of it in modern China.

This documentary (https://www.youtube.com/watch?v=31jXkTLbYFE&feature=youtu.be...) links sushi to the introduction of rice cultivation to Japan. Through China.

And apparently (https://scholarspace.manoa.hawaii.edu/bitstream/10125/17154/...) when it comes to the arrival of rice to Japan, it's not a question of whether or not it came through China, but which of the three possible routes it took.