I woudln't consider that significant at all, rather, insignificant.
15 ppm = fifteen milligrams per kilogram.
So for every kilogram (2.2 pounds) of walnut hulls, whatever, you get 15 mg, provided the information is correct.
If someone wants to ingest 2 pounds of something to get 15 mg of iodine, that's their choice.
Since we are doing calculations let's look at if 15mg is significant or not. The RDA for iodine is 150MICROgrams. A microgram is one thousandth of a milligram. Therefore the RDA for iodine in grams is 0.00015g, requiring only 2.2g of black walnut hull. So you still want to try to ingest anything for a dose of 15mg of iodine even though this is way more than is required by the body? This is why people end up in trouble so many times with alternative medicines. They tend to think that if a little of something is good then more must be better.