Not very shocking considering that this belief was quite popular among the early Imami community, even so amongst the proto-Sunni majority, and since there existed several other versions. Not only that, but I'm fairly sure they didn't trust a munafiq with the task of faithfully compiling together the entirety of the Qur'an as it was revealed, especially when he's running around burning out other vulgates like his predecessors did to the house of our Noble Lady عليه السلام. That being said, there's much dispute regarding just how these alleged interpolations made their way into Shi'i hadith compilations, some suggesting they're authentic statements of the Imams عليه السلام regarding the tampering of the original text, others claim that some of the narrations are true but are intended to be taken in non-literal manner, and yet others are of the view that the bulk of those ahadith are fabrications, falsely attributed to the Imams عليه السلام with some claiming them to have first arose out of the proto-Sunni masses. Allahu 'Alam. (wasalam)