Not a db specialist by any means, but this seems orthogonal to CAP to me. CAP is about distributed databases, RUM is about the trade-offs between read and write speed and memory; can be single node database or distributed.
Oh, I agree with you. I'm not even saying that I disagree with RUM, but I'm just saying that it's too easy and possibly lazy to apply "Pick Three, Take Two" to other technical problems.