An interesting project. The most ancient shipbuilding techniques are just the opposite of what he used, i.e; the hull was built first and sewn together and then a framework inside built. Since most of the strength would be in the outer hull massive frameworks internally were not necessary.
So even with the tools available at the time, (axes, scrapers and rope) building a large floating box or ark is quite feasible. No rudder or tapered bow needed just water tightness which bitumen would furnish.
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.