Even without internet, many of the scientists of Eastern European extraction were able to share secrets to the Soviets. I don't believe any Operation Paperclip scientists were directly implicated as atomic spies (and there may be some reasons the US wouldn't want to bring attention to that), but plenty of other operations occurred, and plenty of other scientists did in fact share secrets with the Soviets.
Now, with the Internet and strong crypto, it's trivial for Chinese professors to send IP back to collaborators in China. That is the basis of the 1000 Talents Plan (1), one of 200 Chinese "talent recruitment" plans.
This is not at all hypothetical. I used to swim with Kang Zhang, who has done amazing work to cure chlamydial blindness, but also took that IP from the US to China (2). Another ophthalmologist would drive all the way from UCLA (the north end of LA) to San Diego to swim with us. I asked him why. He said it was to keep an eye on Kang: he had more macaques in China to run experiments on than anyone in the US could possibly access.