I think he's saying he only needs one diaper to test it out and see if it's good or not. Of course this is complete bs.
Did you test it during the day, or at night? Was it at work, or at home? Was it tested on a weekday, or the weekend? Did you drink a lot of water that day, or very little? Was your body retaining water that day, or expelling extra water from the day before? Did you find yourself peeing a lot, or in smaller steady bits? What percent capacity did your diaper get wet before you were done testing it. Did you sit down a lot, or were you active at the time? Did any of these occur in combination with the other different circumstances above?
As you see, there are many, many different scenarios a diaper needs to be tested under before you know if it is good or not. It's impossible to find out if a particular diaper is good or not by using just one. Saying otherwise is just fooling yourself.
Heck, I find it difficult to fully test a diaper when I only buy a bag (about 12-18). You really need more like a half case. Lol, one. Yeah right.