Probably more like the test meter hitting its stop: "the actual value could be anything large, we don't have a stick that long to measure it against".

Our own IQ tests break down at about 150ish; anything above that is largely guesswork on the part of the people who wrote the test, due to an inability to calibrate the scale above that point. It seems likely that the Wolfe-DeKok index would suffer a similar problem - if you can't find enough people that smart to measure, you've got nothing meaningful to compare the test results with.