'Real' manufacturers typically do not buy housings that have very distinguishing features that look just like rival's. Indeed, even big OEMs try to make cosmetic changes to try to fool people into thinking that the camera is not a rebrand. If digimerge has the same very distinct housing as 2 chinese companies, I am going to bet that they buy from one of those two companies. Indeed, in China and Taiwan, it's not unusual for these companies to OEM to each other.
Lastly, I agree with your statement, "You can work with them to design your own housing and have them make it exclusively for." Since digimerge evidently has the same exact housing as others, it gives weight to the opposing point - that they are not simply buying a housing and designing / building the rest themselves.