No, it was a joint effort between Simcox and Gilchrist. Simcox had his Arizona Militia that declared unilateral war on Mexico and Gilchrist had some other podunk group. They got together and formed the Minutemen. Gilchrist and Simcox are rarely at the same place. I don't think they see eye to eye on much.
I think you're right. They're co founders of MM, but Gilchrist and Corsi penned the book.