They would be able to do things with materials that we would not be able to reverse engineer without advanced molecular nanotechnology.

Considering how fast we could reverse engineer highly advanced technology. How fast could an isolated and uninformed country reverse engineer our technology?

We can look at the example of China. China had its first nuclear test in 1964. However, a nuclear fission weapon involves purifying uranium isotopes and getting a critical mass. The US had nuclear weapons in 1945. The first soviet test was 1949. The Soviet Union helped with some initial research, design, and production preparations. However, it later withdrew support at the last minute, and China had to rely on itself to complete the bomb. The Soviet Union was also engaged in test ban negotiations with the United States in 1959 in order to relax Soviet-American tensions, directly inhibiting the delivery of a prototype to China. Broader disagreements between Soviet and Chinese communist ideologies escalated mutual criticism. The Soviets responded by withdrawing the delivery of a prototype bomb and over 1,400 Russian advisers and technicians involved in 200 scientific projects in China meant to foster cooperation between the two countries.

There was Soviet and Chinese spying to uncover the secrets. This was a high priority effort to copy critical weapons technology. Twenty years with a lot of help, constant spying for information and a common base of science and technology.