Rothbard thought that moral laws (just like mathematical or chemical laws) could only be discovered, but not made or invented. In other words, the binding force of moral laws is not derived from any human authority. No one can make an action moral or immoral just by saying so, any more than I could make 2+2 equal 5 just by saying so.
Hence, there would be no permanent legislature in a libertarian society. Rothbard suggested that at the biginning of a libertarian society, a kind of constitutional convention might promulgate a libertarian law code (and then presumably dissolve itself). But the law code’s authority would not derive from its being promulgated by the convention, but from its content being in tune with the natural law. (Compare: When I publish a new mathematical theorem, whether or not it ought to be accepted by the mathematical community depends on the soundness of the proof, not on the fact that I published it.)
In an anarcho-capitalist society, competing private arbitration and protection agencies would interpret and enforce the law, but that doesn’t mean they would be the government. A government is defined as a monopolist of law-enforcement.