The recommendation you site is for a pair of fish, a fish that is notoriously territorial. I don't see how that applies to my case. I also don't what "clear sign of stress" you see but his color a activity are fine so your going to be more clear.
My comments that he looks stressed were based on his colour, he looks very pale in your photos and that is a sure sign of stress in this species. If thats just a trick of the light then I apologize but even with a flash Ive never seen my chap look so ghostly. This is another trustworthy care sheet that has a recommendation for a single specimen.
This is the first one on google, it happens to be from a very good forum that provides advice for new fishkeepers.
As far as sources for tank size I find this white paper
written by University of Michigan professors to be much more applicable. The paper is a study of early and adult social behavior in paradise fish. If you read you will see that they use tanks ranging from .5 gallons, 2.3 gallons, 5 gallons and 10 gallons to conduct their experiments.
I'm not sure animal testing labs have the best track record when it comes to animal welfare. Its also not a justification of bad practice to say that someone else does worse, that would just be a race to the bottom. I'm not trying to make you feel bad, you obviously care about your fish but he really would benefit from some extra space.