To quote DxO's site: "Here is just a sample of our current customers:..." with a picture of Nikon's image below it. I think it is fair to say that they have ties to camera and lens manufacturers because they are expressly advertising these businesses as their customers on their site. Paste the link in a browser and look for yourself. This isn't conjecture nor is it conspiracy theory; it is fact.
I created a PDF of the website. See the attached. Opening the page sometimes defaults to their Home page.
Wrong. It means that several large companies find enough value in DxOMark products and services that they are willing to purchase them, and Nikon happens to be one of many. It's not advertising for Nikon, it's advertising for DxOMark. A lot of large companies list who their customers are, so what? It provides a reassurance for other companies who are looking at purchasing their equipment and services, and therefore further
establishes their legitimacy. It doesn't make DxOMark beholden to Nikon in any way.
A customer is a customer, either they find value in your products and services, or they don't. Nikon currently finds that DxoMark's equipment and procedures are helpful in their quest to make better sensors. If anything this makes Nikon beholden to DxOMark, not the other way around.