I know the definition of genre. I object to your usage of shonen, in that it only applies to a particular subset of things actually aimed at the demographic, and includes things that are actually outside the genre. That is, the word shonen, in common parlance, does not mean something so specific.
Last time I checked, advertisements aren't published manga. I suppose that they ARE shonen adverts, since that's who they want to buy the stuff, though.
So no, your argument is crap. You're trying to argue that the word shonen means a specific story type, which is proved wrong by having just ONE thing that isn't. The fact that so many slice of life comedies shonen really doesn't help your argument that saying shonen genre should mean anything but the demographic.
EDIT: Bakuman is done by one of the two people that worked on Death Note. That's all I know.