The word anarchy comes from the word anarchia, which combines a, meaning "not, without" and arkhi, meaning "ruler, leader, authority." Thus refering to a person or society "without rulers" or "without leaders".
Voluntaryism is a "form" of anarchism, but it's not the only possible outcome.
And I'm saying this as a voluntaryist. It's just important to stay honest and true, because people who oppose us are quick to point out erroneus stuff.