Don't know where you're getting your information from, but all the data and studies are out there and easily accessible. And all the information about lithium battery chemistry is out there and accessible. And every single one says that maintaining the maximum voltage, aka a 100% charge degrades the battery. Enough so that charging only to 75% can double the battery life, and charging to 85% can extend it by 50%.
So it doesn't matter what you and your mate experience, anecdotal evidence is no substitute to all the literature out there, all of which say your information is wrong. You're one man trying to argue against 100s if not 1000s of peer reviewed articles, and engineers designing batteries and devices, all because of what you experienced with your phone, whilst collecting absolutely 0 data, conducting 0 studies, and 0 research.
A simple Google search goes along way... Or check out the link i sent in my first reply. Lots of information there on battery health, based on actual lithium battery science, and studies. Not peoples opinions and personal experiences.