About twice a year “protesters” from Hong Kong and the mainland try to land on those island to reclaim them for China.
You are confusing two different disputes. You are thinking of the Senkaku islands far south of Japan, towards Taiwan. That is where China claims and the protesters land. This dispute is referring to the Kurile islands, north of Hokkaido (northernmost main island) that Russia took at the end of WWII.
Two different disputes.
Those islands (located in east China Sea) are not the part of the Northern territories dispute between Russia-Japan.