I disagree. Yes, failing to define the category means that books not intended by the nominator might be eligible. No, this is not a problem. If a couple of back-and-forth posts don't clear up the matter and reach a simple consensus, then we have... categories with disagreements about what belongs in them.
And anyone can nominate a book they believe fits the category, and if it gets two supporting votes, it'll be voted on. That means three people have to agree it fits the category. To be *selected,* many people would have to agree that it not only fits the category, but is something they'd like to read and discuss.
I don't think mandatory definitions in advance are required to keep out-of-genre books away from the monthly selection process.
(It is possible to define Classics, in several different ways. So far, the MR community has mostly declined to do so, preferring to allow multiple sometimes-conflicting definitions to inspire the nominations. This happens in SF and Fantasy too.)