How easy would it have been for Platinum to artificially extend this game's length? Take away the Ninja Run, for starters, so you can't move at a ridiculously awesome speed whenever you want. Count the time you watch cutscenes and codecs and replay a difficult fight, all on your overall time. Don't allow codecs to be skipped, and force more of them on you. I could go on.
God bless Platinum Games for caring about gameplay first, and making things that are fun and wild and stylish and a freaking blast, and not giving two shits about stupid bullet points that marketing teams and publishers force every other developer to focus on, because it appeals to dumb people who only value games for "price-divided-by-time-played." Had this been anyone else, all of the above (and more) would have happened, and we'd have heard for the last six months how this is a twelve hour gameplay experience, only for people to play it and say "eh, more like 7 or 8." Oh, and the game wouldn't be as fun.