Meh I don't see how it's fantastic marketing. Firstly, I expect about 95% of people who buy NV will already have heard of, and most likely invested a significant amount of time playing, at least one Elder Scrolls game [most likely Oblivion]. So it's not as if they'd be reaching a demographic that wasn't already aware of the series. In fact, many or even most people who buy NV are probably already anticipating TES V to some degree... So it doesn't make sense to divert money/resources into funding an ad campaign that will only be seen by people who are already virtual locks to buy the product. Plus, that ad would be all over the internet within seconds, and IMO it'd be a very anticlimactic way of revealing/announcing the game that some of us have been anticipating for half a decade, or more (for people who thought Oblivion failed to live up to its predecessors).
Only if that was the only TES:V ad they ever made. For the very first ad, sneakily placing it somewhere where some of their biggest fans will find it, on a manual that will be produced regardless of whether the ad is there or not, makes a lot of sense actually. No significant extra costs at all, directed to some of their biggest fans first and foremost before launching into a full ad campaign to reach the masses, and you're right - the internet would go crazy with the news. It's a perfect strategy, for all the reasons you claimed it would be awful.
And anti-climatic? Really? You'd pick up that manual, see a TES:V symbol on the back, and sigh? Are you expecting to see it carved onto the face of the moon when you get up to close the curtains one night?