Good video. Good ol' reliable Youtube.
Seems pretty clear from the video that it is completely fair to say the engine is hand assembled. The only thing computers are doing is sealants, torquing, component tracking and data collection (of course as well as 99%+ of the component production...). Again based on the relatively low volume of engines being produced this is THE least expensive way to build these engines. MB is absolutely taking advantage of this necessity (not choice) and appealing to the race engine/hot rodding/magic of the human hands angles to market the hand built "feature". If these engines were made in substantially higher volumes they WOULD be made faster, more precisely and at a lower cost by moving away from hand built to robot/machine/computer built.