Well I can’t speak to your experience with your brother, but I spoke of “philosophers” plural, and contrasted that with - quote - “a software course [our friend here] took one time”. Perhaps your brother isn’t great with mind and language, but that doesn’t mean that even he is so incompetent that he can’t do better than our target here. For all that philosophers plural, or this one philosopher, have hit stumbling blocks along the way, they have made an attempt to more than simply stipulate a wildly counter-intuitive and pragmatically tendentious meaning for this complicated word “belief” (indeed: “doxa”).

I don’t know where you get the idea that “we” have “no actual understanding” of language and mind, however, because at least philosophers (as well as their interdisciplinary friends in some of the sciences) have quite a lot of understanding of language and mind, and especially language. Since the innovations of Gottlob Frege, for example, the interpretation of semantics according to a logic of truth has been extremely helpful in clarifying how sentences bear relations to not just external but reality in general. Linguists have done extensive work on the pragmatics of language, which fills out this picture to make sense not just of propositional but questioning and commanding sentences.

These are just examples, there is obviously also a lot more.