Definitely a lack of clear info on the website. I just took a look at one of the sailings with the 'Free Ocean Voyage' offer, and this paragraph is tucked away between the route map and cabin descriptions:
Free ocean Voyage before or after your cruise
For any cruise bought in combination with an Ocean Voyage, your package (Cruise + Ocean Voyage) becomes non-refundable and non-changeable. The combined offer (Cruise + free Ocean Voyage) is non-retroactive for bookings made before the offer. Safety port taxes and fees are not included. This offer is subject to availability, and applies exclusively to a cabin category equivalent to the cabin already booked.
So it seems pretty much in line with my previous post, with the added 'bonus' of ultra-restrictive T&Cs re: cancellation, etc. Still, for those of us who love sea days it would be a no-brainer to take advantage of this extra time on board 🙂