And from what I understand, anarcho-capitalism is basically short for Anarcho-Free Market. Free market being synonymous with voluntary market...
Here Rothbard defines what he means when he says 'capitalism':
The difference between free-market capitalism and state capitalism is precisely the difference between, on the one hand, peaceful, voluntary exchange, and on the other, violent expropriation.
So using Rothbard's definition of anarchy (absence of the State, not government), and combine it with free-market capitalism (the voluntary market), anarcho-capitalism is the same as voluntaryism.
The reason I bring this up is because I don't know of any anarcho-capitalist who does not support voluntary government/governance (voluntaryism). ( part 3 coming lol)