One of the articles you linked just proves my point that he's using it wrong and that thou and you were used at the same time for different things.
There seems to be a bit of doubling-up there, but I've always generally heard that "thee" and "thou" were used for strangers and inferiors, while "ye" and "you" were used for equals and superiors. Considering the language of the setting, including Urianger's "older Eorzean" has moved on well past 1300s English, "you" as plural in the context of the way Urianger talks doesn't work anymore. It also doesn't make sense for him to just use "thou" for almost every situation and feels more like the writers are just trying to make him sound like he's trying to sound old fashioned.Starting in the 1300s, thou and thee were used to express familiarity, formality, contempt, for addressing strangers, superiors, inferiors, or in situations when indicating singularity to avoid confusion was needed; concurrently, the plural forms, ye and you began to also be used for singular: typically for addressing rulers, superiors, equals, inferiors, parents, younger persons, and significant others.