You are correct, the video was not entirely about Smash Bros. However, it was about E3 -- an event where Smash Bros information is known to be announced -- a Smash Bros demo, and a Smash bros tourny. Obviously the video is largely about Smash Bros, even if it is about other things too.
You are right that the guardian's theme hardly matters, because the guardians are not characters who would cause commotion in the fanbase -- nobody really cares about them. They can use the guardian's music however they wish because using it does not imply anything other than Skyward Sword representation. Ghirahim, on the other hand, is considered to be a likely newcomer candidate. Using his theme in official Nintendo videos is obviously going to cause some sparks.
So what if the target audience is America? Are you implying that a hint can't just be directed at one country? A very, very important country, at that. Not every hint has to be shown to fans in every country. I would search for examples, but that seems unnecessary, as it should be obvious without them. Your logic is flimsy, Mr. Wright.