If we're assuming Doma is Japan-based simply because of ninjas, well, assassin's are hardly unique to Japan. As for people attributing the yukatas to Doma, I'd say it's more likely they came from wherever the Heavensturn people originated from, and that was (I think) never said to be Doma. And I'd certainly think it wasn't, because with Doma being purged by the Empire, I doubt there are a lot of Doman tourists wandering around.
