This is a very good guess, and I believe it's precisely how it's done. Doing anything diff-ing the color channels still requires a lossy transformation step, so it's not quite as "raw."

And you're right, that treating it as a single monochrome image makes it less compressible, because you get lots of high frequency information between adjacent pixels with different color. Which is sort of the opposite of what you want.

Note also that, despite calling it "raw," Red's implementation is lossily compressed. You can even tune the compression ratio of their "raw" codec, and back in the very early days, there was certain subject matter that just looked terrible coming out of the camera, even as raw -- I remember one particular tree that an early user would film to test new firmware updates; in the earliest firmware builds, it actually crashed the camera (!). The images it did produce showed tons of wavelet compression artifacts.