The easiest way to say it is, "Shore excursions sold by DCL." If the cruise line sells it, the cruise line stands behind it.
As has been noted, very few excursions are actually operated by DCL, and that's true of nearly every cruise line. It's just not financially wise to train and carry a shore excursion staff on board the ship, or to have port-based operations and facilities that won't be kept busy every day of the week.
With contractors, if the ship has to skip the port, the cruise line loses nothing. If fewer guests are interested in a particular excursion this visit, the problem is the contractor's, not the cruise line's. Even on Castaway Cay, some of the experiences are run by contractors.
Co-Author, PassPorter's Walt Disney World, PassPorter's Disney Cruise Line, and PassPorter's Disneyland and Southern California Attractions