EDIT: I suspect you already know all of this. Your article seems to indicate. SO you can ignore this or take it as me adding my own words.
Anarchy does not mean violent. That is what it has been hijacked to mean. A lot of people that use the term think of violence, chaos, and scenes from road warrior.
It is actually a derivative of the word An-archos which means no RULER.
This does not mean there cannot be leaders. It simply means that no one has the right to RULE you. You can voluntarily choose to follow someone for however or whatever you want which is the difference between a LEADER and a RULER.
So it isn't a contradiction of terms at all. However, because of the people that do claim to be anarchists and are violent, and the media spin on that many Anarcho-Capitalists simply call themselves Voluntaryists now.
Anarcho-Capitalism essentially means no rulers + free market/laissez faire.
This is based upon the original meanings. Not the popular meanings created by propaganda and media.