My understanding is that CMYK here indicates the color model of the image data after the conversion, and not an additional conversion. In other words, Marti is assuming that the printer profile is a CMYK profile (which, as you correctly pointed out, is not always the case, as printer profiles can also be RGB).
Then the data is converted back from printer to CIELab.
Again, my understanding is that the last RGB is not an additional conversion, but just a “reminder” that the output of the conversion to the monitor profile is in RGB space.
Clearly, this view of the soft-proofing chain is very printer-centered, and does not consider other output media (like web publishing in sRGB colorspace). Again, Elle already pointed this out.
Another point that should be made clear is that soft-proofing can be very misleading in some situations, at least unless you have an high-end wide-gamut display. Modern inkjet printers can output colors well outside of the sRGB gamut. So, if you are soft-proofing to a standard sRGB display you will actually never see ALL the colors of your final print. For example, your printer will likely output highly saturated greens and yellows that your monitor is not capable of displaying. In such a situation, soft-proofing is unreliable 
.
However, there is another tool that can give you good guidelines independently of the display you are using: the gamut warning. This tool usually displays with a uniform solid color (gray for example) the colors that are outside of the gamut of the proofed profile. This can be used as a guideline to further edit the image and bring the over-saturated colors back into the output gamut, if that is what you are aiming for. And the feedback you get is much more accurate that the visual output of a soft-proofing chain…
EDIT: in other words, if the goal of soft-proofing is to check if the image colors fit into the output gamut, then the gamut warning is way more accurate and intuitive than a soft-proofing setup. On the other hand, if the goal is to “see how the image will look once printed on a specific paper”, then soft-proofing is the way to go. However, in this case one most likely needs a wide-gamut display that covers a large fraction of the printer gamut, and additional options in the soft-proofing set-up. The “simulate black ink” and “simulate paper color” that are included in PhotoFlow’s soft-proofing code are exactly meant for this…