That's just cold war propoganda. They had four B-29s that they cross-referenced in the reverse engineering process, so patches and mis production issues that only affected one were left off.

They did go to great lengths to clone though, but that was a "we can do exactly what you can in two years from essentially nothing" international politics dick measuring contest. Anything other than an exact clone would lead to a "look at how the commies can only make an inferior clone" talking point.