actually it is, because there is no set gaming definition for person as their is in the literary sense.
Typically game camera can be called a number of things, but a FPS is always a FPS, a TPS typically refers to an over the shoulder shooter, however to say that metal slug is not in the third person is rather narrow minded, considering the game terminology was taken from the literary sense.
If you want over the should third person, then say that, but don't tell me that a side scroller isn't third person.
Even "god" view, typically used in sim-city and such COULD be stretched to third, but I wouldn't go there because you're typically not basing those game around a single protagonist.
Hell, Epic Game's Cliffy B referred to the camera in GoW as 2nd person, do you even want to go into the specifics of how impossible that is? well not impossible, I guess you theoretically could play the game through the eyes of your partner, but control would be a little odd don't you think?
Oh and from your own linked article...
Today platformers remain a vital niche genre, but have never recaptured the kind of popularity that they held in past years. In 1998, Platform games had a 15% share of the market (and even higher during their heyday), but only four years later that figure had dropped to 2%. Even the much acclaimed Psychonauts experienced modest sales of a little more than 85,000 units across all platforms, and franchises like the once mighty Tomb Raider began to sag. Other forms of third person action games have cut into the sales of platformers as RPGs and First Person Shooters have grown in selling power. One might also say that this decline is an inevitable result of a larger and more diverse market than in years past, as no one genre has managed to achieve the same kind of dominance that platform games did in the 8 and 16-bit console
Note the second to last sentence... "other third person action games have..." i.e. in addition to the third person platformer.
A platformer is a genre title, not a camera definition. You could have a first person platformer too, or like tomb raider or many other similar games, a variation on the location of the camera, but still 3rd person.
And lastly, you again are basing your view of what titles should be on the Wii on your own bias. Just because you don't like a game, doesn't mean other don't. Very many people absolutely love(d) Metal Slug and all the side scrolling games like it. Your opinion is not more important than anyone elses.
Just because the system is capable of revolutionary control, doesn't mean it should ONLY have that type of control. This isn't an either/or debate. It's not metal slug or some new amazing sword fighting sim. it can be both at no cost to the other title.