Bizarrely the Soviets and Americans were close enough

The implication was always that the Sino-Soviet split caused the USA to choose the USSR over China. The Soviet Union in black isle's fallout was more like "successful" revisionist post-glasnost state that diminish in its power - which Bethesda kinda forgot and given how fallout 4 was written when the whole crimea thing was popping off, the USSR became a useful enemy to use again.