That's like saying pizza isn't Italian. Pizza Hut isn't really Italian, but the roots are there. From the article:

JIANG: Salmon sushi started showing up all over Japan, especially at the cheap sushi restaurants, the ones with the conveyor belts. Bjorn realized salmon sushi had become a thing when a few years later, he was walking around Tokyo and he noticed the little plastic sushi replicas in the restaurant windows.

It may not have started that way, but it certainly is that way now.