Revisionism in my view being ideology that leads to or allows the expansion and existence of private property.

Private property is not eternally bad or good, and capitalism is not simply private ownership or commodity production; the “superstructure” is not just a product of the “base”, but reinforces it and shapes it onwards, as Engels said, not cause and effect but reciprocation. You cannot split the dialectic apart (although sometimes it’s necessary for explanation), as people try to do with imperialism, narrowing Lenin’s “definition” to exclude the political and falling into vulgar materialism. China has private property, but it is itself public property extended, and the state in its ideological and practical spheres has shown itself to be a state of the workers.

If you read Anti-Dühring, Engels gives insight into the negation of the negation, that there are special conditions required for the sublation of the negation, i.e. you can negate capitalism by destructive madness, but you make it impossible to sublate this negation, and so everything resets. The negation of primitive classless society provides the groundwork for its own negation, advanced classless society (communism) by expanding production/socialization (put simply). There is no way you can get to socialism without private property, commodity production, etc., as Lenin says: “Socialism is inconceivable without large-scale capitalist engineering based on the latest discoveries of modern science” (The Tax in Kind).

Or Marx: “even when a society has got upon the right track for the discovery of the natural laws of its movement — and it is the ultimate aim of this work, to lay bare the economic law of motion of modern society — it can neither clear by bold leaps, nor remove by legal enactments, the obstacles offered by the successive phases of its normal development. But it can shorten and lessen the birth-pangs” (Capital Vol. 1).

Leninism, emerging out of imperialism, showed that communists could take power where imperialism was most concentrated, so mainly less developed countries, and this was the case with China. Mao himself acknowledged that private property was necessary for China’s development: “In the new-democratic republic under the leadership of the proletariat, the state enterprises will be of a socialist character and will constitute the leading force in the whole national economy, but the republic will neither confiscate capitalist private property in general nor forbid the development of such capitalist production as does not ‘dominate the livelihood of the people’, for China's economy is still very backward” (On New Democracy].

China and the USSR had no choice but to not only let private property exist but grow in some cases, of course while maintaining control over it, and you point to the fact that the NEP

was very very short

But as you say it was Stalin who ended it, not Lenin, and I think maybe that was a mistake, but maybe it wasn’t. In The Tax in Kind, Lenin talks about transitioning from a wartime economy, but it’s lucky this transition was reeled back, because later the USSR would be threatened by Hitler’s colonial invasion, and things like collective farming would play a big part in the Soviet victory against fascism.