[tearidrusydet]

I think if you actually believe that Abdullah ibn Mas`ud didn't know that those surahs were part of the Quran then you have no real reason to believe in any Hadith, or anything else for that matter, transmitted by him, as even the most ignorant Muslim today knows this.

The details of the issue here are irrelevant. The only relevance is of the Ijmaa opposing the view of Ibn Masood ra on the mentioned issue. Kindly read the classical scholars about this issue.
Hafiz Bazzar after relating these traditions of Ibn Mas’ud in his Musnad, has written that he is solitary and isolated in his this opinion; no one from among the Companions has supported this view.