I know from previous tests that I can’t hear above 18.5Khz, but I can clearly hear the 20Khz from this site because of the resulting lower frequency distortions.

Conclusion: you can’t just download these tones and play them back on the your computer reliably, the hardware and software cause distortions in the tone which are lower frequency than the original files and hence highly audible.

Also the tones need to be tapered to stop the annoying click at the beginning and end of the tone.

First of, the acoustic of you room will change everything.
Second, speakers and headphones have different frequency ranges, and will also have tip and dips in the frequency bands. Meaning that for a certain headphone/speaker compare to the others, it might reproduce a lot more of a frequency than an other, and might have a strong dip at an other. Which is not helped (worsen) by the room acoustic.
Third, there’s the body from the start. Not everyone is born with the same hearing proficiency. Some will NEVER hear over 15KHZ no matter how young or old they are.

So don’t get to much rave or anger about this product.