There are many things other than crops that are needed in order for a society to become truly advanced. After all, there were farming societies in the Americas and in New Guinea, among other places, and the Europeans conquered them all. Therefore, crops are not all that is needed.
The main thing that is needed outside of crops is technology. Both the word “guns” and the word “steel” from the title of the book have to do with technology. This implies that technology is the most important thing for societies to acquire. But what is needed in order for societies to get technology? Let us look at two factors that are important.
One factor is time. It takes a long time for a society to develop good technology. This means that a society that gets farming early is likely to be more powerful than a society that gets farming late. This is one reason why Native American farmers were not able to defeat European farmers. The Europeans had been farming much longer and had therefore developed more technology.
A second factor is the potential for diffusion of technology. It is hard for a society to invent everything it needs on its own. It is much easier to get a lot of technology if you invent some of it and borrow other kinds of technology from neighboring societies. This means that it is important to have neighboring societies that have agriculture. This is something that, for example, the highlanders in New Guinea did not have. By contrast, the Europeans could and did borrow technology from all over Eurasia.
So, two things that are needed, other than crops, are time and the potential for the diffusion of technology.