Thanks to a chat with Abby, I solved the problem. The yarn required for the ribwarmer is a heavy, heavy worsted.
I just swatched with Brown Sheep Lamb's Pride worsted on 5mm needles and it matches the cable pattern gauge.
The rib pattern gauge has got to be the misprint.
Wahoo! Off to buy some more yarn! The good news is I know it will knit up very quickly.