John Dark here - I'm the publisher of the site, and welcome any questions. The short history is that I started the site when I noticed that almost all of my system was made locally, and I became curious about the state of American high-end audio manufacturing. There are great components made all over the world, but I live in the US and wanted to know more about the companies doing business near me. I was surprised to find so many manufacturers here. I'm sure I've missed a few companies (there are over 200 listed and categorized), and sure to be more.
I list companies that can lay claim to "Made in USA" and "Assembled in USA" using the FTC guidelines as I can best interpret them, and as companies make information available. I don't list companies that misuse those tags, or that design and distribute from the US but import their goods. You can learn more here: https://americanmadeaudio.com/2016/06/25/made-usa-standard-guidelines-audio-equipment-manufacturers-consumers/
As for Benchmark, either I missed them or they weren't as clear that they assemble in the USA when I first created the list. I've just added them to the site. I'm less familiar with the other brands listed above. The best way to get on the list is to have a representative from that company email the address in the "About Us" page, but I'll add companies from time to time as I find them.