He’s making the same nod to conventional expectation of a “ship” that countless illustrators of books containing reference to the ark of Noah have. So, it has a bow and a rudder.
The directions contained within the Bible lead to a large floating box, as you note, the very specific proportions of which have been tested in the modern era and shown to be ideal for remaining upright and afloat in very rough seas. That’s enough. But, people expect to see what they expect to see, and have been accomodated.
I’ve seen and heard the floating box theory before, but there’s a reason why modern barges (i.e. floating boxes) aren’t taken into the open ocean.