Yeah, I definitely don't think that this would give you mathematically provable security, especially if you are including n-grams which would allow you to chain

Yeah, I definitely don't think that this would give you mathematically provable security, especially if you are including n-grams which would allow you to chain together sentences combined with a language model. How much that matters in practice depends on the dimensionality reduction. In the context of google I doubt if this is even much of a specific goal, since no matter what they train on, they actually have access to the source text if they want it.