That's not likely to happen, as long as the survey was conducted in Japanese.
* **USA** → , {}
As like most nations outside US do, we prefer to abbreviate the country's name to toponym.
* **USSR** → {} ()
* **Russia** →
* ( **"united states"** → {})
If someone should believe that the Soviet Union was America, then it's another failure of public education, but irrelevant to linguistic matters.
**PS**
I Googled for Japanese source of "25 per cent of young Japanese people believe that the bombings were carried out by the USSR" but I could find nothing.