MDF first then veneer, not plywood. Almost all plywood has core voids, even pricey Baltic Birch. Unless you specify all B grade veneer, but even those will have defects in the veneer faces.
B&W uses wood veneer sheets to build their enclosures, probably 1/10" or less, maybe even 1.5mm sheets, so yes using 12-13 sheets will get you 19mm or 3/4" close. Using veneer sheets allow them to bend the form to create their desired shape.
MDF is homogeneous, same density from face to back, all throughout, no core voids. Can be moulded and shaped easily, painted and or overlaid with wood veneer and stained for a super high end finish. The boxes using MDF are perfectly matched meaning both speakers will be identical to within set tolerances of probably +/- .03125".