This is a good question, because much of what steel does seems to be the same as what the guns do. But Diamond is making somewhat of a distinction between the two.
To Diamond, the "steel" part of the title refers to technology that is not necessarily military technology. Here and there in the text he refers to the importance of steel tools. These tools made it possible for the dominant cultures to create things such as modern ships on which they could cross the seas to conquer the societies that did not have the same level of technology.
So, while military weapons are a form of technology, they are not the only form. Diamond is using the term "steel" to refer to other kinds of technology like plows and other sorts of tools that help the dominant societies gain power.