Keep in mind that is not a manufacturer's website. If you go look those products up on the manufacturer website(s), I don't think you'll find any "50 percent" language. I sure haven't. The capacity of the extension is as marked on the extension, 350lbs tongue, 3500lbs trailer.
With that information in hand, my conclusion is this 50 percent rule is something etrailer.com made up for CYA purposes, and/or perhaps to reduce the number of questions on the product.
My guess here is that inches matter, but not nearly as much as etrailer.com makes it out to be. For sure you know the reduction is no more than 2.77 percent per inch, as that is what etrailer.com's claim on the 18" extension works out to be. As an engineer and only looking at it from a 500' view, I would be inclined to think that the reduction is closer to 2 percent per inch, if not less, but it also may not be linear.
