That is false. They were a purely NZ operation launching sub-orbital rockets before they got into DARPA contracts. What you meant to say is "Before they ever launched Electron", and I'm pretty sure that is false too, they weren't "primarily" American, the majority of the workforce was in NZ until years after that.
I'm 99% sure that they went to the US venture to raise money to build Electron and that's when they made the US company that primary and the NZ company the subsidiary.
SpaceX has most staff in California.
It's a Texas company.