You're really just renting them and then they use those funds towards the cruise. Using points to trade for cruises is a poor value so this way you get a higher value for the points.
We didn't book our cruise through David's, but we did use them to rent out our points and funded our cruise that way. We booked the cruise through Costco since that made the most sense to us, but I'm sure booking through David's is just fine.
Booking a cruise with points is not a very equitable trade, you can get much more cruise for your points by renting them out to fund the cruise.
Not an accountant, but I don't believe a swap is taxable. It could become taxable if there is a gain on it. i.e., if you rent the points for more money than the DCL cruise, you'd have to pay tax on the extra money returned to you.