Don't US companies source a lot of parts from overseas? Ford even has plants in Mexico. US and Japanese companies often share designs. The boundary between foreign and domestic is a lot blurrier than it used to be. This point here is mostly about the management of these various companies, not so much their location or operations.

Also, the article mentions specifically that Toyota and Honda use a lot of American-made parts, which are of higher quality than whatever Ford/GM/Chrysler are using (doesn't say where they are getting parts from) and that is a major contributor to the difference.