Pedagogical history isn't a strong argument for something's truth, just its usefulness to someone. Theology was considered a key component of a higher education for centuries and is still taught for example.

People have always strongly believed, and taught, things about their own languages and languages in general. They're not all wrong necessarily. But linguistics over the last like 80 years has produced some models that have incredible predictive and explanatory power, and are incompatible with a view of an objective "correct" or "best" or "standard" form of a language.