I think they should measure these ships by underwater volume at time of departure.
Otherwise, the next 'yacht' will be a foot long and wide but 20 feet high, most of it underwater, and some brave/suicidal soul will volunteer to 'sail' it.
You can also be clever by designing a 'boat' that is nothing more than a mast with a large counterweight standing vertically in the water, attach your permanently inflated 20 meter long 'life raft' to it, and live out of that life raft during the trip. Such a 'boat' could easily be less than a foot long.
I think the person designing this boat would also call your hypothetical cheating, but that doesn't make his boat cheating.
I would not be surprised to find that there is a separate record for lowest displacement crossing, though that would be much more dangerous as it would encourage you to skimp on supplies.
In any event this boat is pretty lean by your standard too, as at 1800lbs (~800kg) laden, it displaces about 5-6 times as much as a fully rigged and crewed Laser Standard[1], which is not something you could cross the Atlantic in.
Underwater volume == weight of the boat. You need a hefty underwater keel in a sailboat, especially a blue-water boat, otherwise the forces on the sail could make it capsize.