It's not so much assumption as extrapolation. Not many of the ship building techniques used 200 or 300 years ago we used in the construction of any modern ship, jet or space craft. So it's not so much of a stretch to expect that none of the techniques used today would be used to build the Enterprise.
Actually they are: The way we make ships is pretty much the same method, some of the materials have changed but the basic construction (done on dry land) is the same. Bolts are put in place paint is laid on the keel is laid out on ribs. Really making a sea ship hasn't radically changed from a hundred years ago - now they just add engines...