In the Edo period, Hokkaidō was assigned to the provincial government of the Matsumae clan. Ainu were forced into laboring for one-seventh to one-fifth pay by Matsumae retainers, which the Ainu regarded as slavery.
The historical context of this passage is that during the Edo period in Japan, the Ainu people were subject to forced labor and exploitation by the Matsumae clan's retainers. They viewed this as a form of slavery.