Er no you cant claim the Soviet Union was the only legitimate successor state to the Russian Empire In their various ways Poland Lithuania Latvia Estonia and Finland were equally successor states as well as the other republics which failed to survive such as Menshevik Georgia Dashnak Armenia the Azerbaijan Democratic Republic the Belarusian National Republic the various independent versions of Ukraine etc Likewise you cant say the only successor states to the AustroHungarian Empire were Austria and Hungary and omit Czechoslovakia Yugoslavia and Romania Not in an encyclopaedia you cant anyway