DY: The Rambam said it. Many Achronim go out of their way to prove we don’t hold like him. But he very clearly said it. Also, if you read his letters, you see that he explains that he didn’t do that.
And the Mishnah B’rurah is clearly accepted on many things, mostly due to being the most convenient resource. There are many places with many Minhagim otherwise. My father told me he has seen with his own eyes where Shuls and communities have changed their Minhagim because the Rav didn’t know their Minhag so just went with the Mishnah B’rurah when anyone in the community would have told you that the Minhag is otherwise (e.g. who Davens for the Amud when there’s a Yahrtzeit and a Shloshim in the Shul). I won’t say that you’re doing wrong if you’re going with the M”B, Chas V’shalom. But I think it is quite a stretch to say that all of K’lal Yisrael has accepted all of the Mishnah B’rurah. If that were true, no one would ever learn any other Halachah Seforim anymore. Also, the vast majority of people don’t know how to properly read the Mishnah B’rurah or what to do when they find a Stirah.