I'll just return a bit to the kappa device to say, it's coul be that the Akyuu learned about it during on of the kappa bazars that she mentions. We don't really know the extent of Marisa's influence in the writting of the book, after all.
Back to Marisa's lying... like Akyuu's mistakes, misinformations and exaggerations, it's pretty much obvious when it happens. When I made my "Marisa doesn't lie all the time", it was more in response to Setsakura implying that SoPM is unreliable solely because of its authors; Marisa isn't really show to lie about descriptions of danmaku and spell cards, for instance, so why would she lie about a list of items that the kappa have? Doesn't make sense, and nothing points towards that.