No, it doesn’t make the claim true. To illustrate, take another example of a false belief: “Drinking cold liquids can give you a common cold.” This is scientifically false regardless of how many people believe it. If a book author wrote, “Some parts of Nepali society see cold drinks as a cause of the common cold,” that does not make the belief itself true. It does not make the belief worthy of serious debate. In the same vein, “Madhesis are Indians” can be believed by some parts of Nepali society, yet still be false. I hope this clarifies the issue.