If you waved it in Korea or China then yes I’m sure it would be offensive. It was waved in England. The Japanese like it. I assume it was waved for Mitoma. I expect he liked it. Surely that’s the point?
Unfair to compare it to the Swastika. It’s still often used in Japan. Admittedly you’re right about Korea not liking it. But neither do the Chinese, so ups and downs.
Wiki
At present, the flag is flown by the Japan Maritime Self-Defense Force, and an eight-ray version is flown by the Japan...