This is not surprising at all. The first bombs were made when nuclear physics was not well understood, and the Las Almos computer, was a group of humans working in an assembly line to perform complex calculations by hand. These calculations were required to understand the interaction of the various pieces.

A few decades later, the theories of nuclear physics had advanced considerably, IC computers were available for rapid calculations, the algorithms required for these calculations were present in textbooks, as were the results of many computations that did not have to be redone.

Today, designing every aspect of a nuclear weapon, from the actual weapon itself to the machines and factories required to manufacture them, is even easier. The only real deterrent to countries building nuclear weapons are political in nature. Probably over a hundred countries could do it very rapidly if they wanted to, but thankfully we are limited to about a dozen of them.

This is something to reflect on, when arguing that the spread of bad technology, especially one that does require many millions to manufacture, cannot be stopped.