So this is one of those "I guess it sorta is true from a certain point of view" things that doesn't really account for how the situation really worked.

So lets get one thing out of the way first. The Japanese when surrendering to the joint Allies surrendered to the USSR too. There was a Soviet representative aboard the Missouri in Tokyo Bay who signed the document.

But that simply ended hostilities and set up the Allied(US) run occupation of the islands under the Supreme Commander of Allied Powers(MacArthur) and the International Military Tribunal for the Far East and left many other issues unresolved.

These would be hammered out in 1951 in what would eventually become the Treaty of San Francisco. It ended Allied Occupation, acknowledged territorial changes for the most part, the war crimes trial results. However while the USSR attended they were unhappy with the process and other provisions, including not giving them complete control of several disputed islands, and the exclusion of the PRC. Thus they refused to sign the treaty and went on their own way.

However in 1956 a joint Japanese-Soviet Declaration confirmed the state of peace between the nations and established an exchange of ministers. However since then no formal treaty has been signed to take it another step beyond, and disputes over the Kuril Islands among others remain a divisive topic.