>almost as old as guitar-family instruments.

They are as old as we can trace it back, vihuela/viola de mano were multi-course, as was the baroque guitar which developed from them. The romantic guitar brought us single string courses and in the guitar family double courses died out other than in a few nooks and crannies. I don't think this has any effect on what TFA proposes and the author is well aware of this bit of information, but it has been years since I read it and maybe I am forgetting something?