I believe the skepticism of Peptest is that it detects reflux when it reaches the upper digestive tract and/or lungs, but not necessarily detects reflux when it is limited to the lower digestive tract. So you could have reflux, but if it's not refluxing high enough up the esophagus, the Peptest won't detect it and would give a false negative result.
I guess, but what about
when I literally taste the "acid" or post nasal drip, or when I feel something emerge in my mouth; shouldn't it be valid then too? :p