I called them and talked to the rep.
The thing on Amazon might be a generic type of description. Or it may be that they consider a certain amount of the elastic is generally used to hold it in place.
But if you think about it, if you detach the slides and cut some of the elastic to shorten it, and then reattach the slides, you can get it all the way down to 8" if you wanted (the length of the actual pouch).
If you are using it for a 20" waist, you could have the 8" pouch, plus 12" of elastic (6" on each side of the pouch) to still give you some elasticity for comfort.
I have modified the heck out of many SpiBelts for races. They have a double pocket one, but I made a triple pocket one to hold enough stuff.
If you get it from Amazon and you don’t think it will work, you can send it back for a refund.
I think SpiBelt would be a good thing to start with and customize if you are handy with sewing.