I read that too, but then I read their choices for the backstrap. I would want the "Flat" option. From that description, it would seem that either adding a magwell isn't possible, or they don't actually remove the hump as much as they say they do.
BACKSTRAP REDUCTION
None - No backstrap reduction performed on the frame. Maintains factory backstrap angle as well as factory feel in hand.
50% - Reduction of the hump on the backstrap of the frame by half before texturing, reducing some of the overall girth of the firearm and adding to ease of sight acquisition.
Flat - Reduction of the hump on the backstrap of the frame making it straight flat line from the beavertail section to the bottom of the frame prior to texturing. Reducing the overall girth of the frame and adding to ease of sight acquisition.