More like game journalism is a crappy medium. Seriously, there is little to no fact checking amongst any of the major gaming press sites, even IGN, as large as they are. There have been many instances of false rumors perpetrated throughout places like Gonintendo (one of the worst offenders of this), IGN, Gamespot, 1up, etc.
Fact checking in which sense? Rumors are rumors and are definitely marketed AS rumors most of the time.
This isn't simply a rumor, this is a continuation of a series of news stories that has been leading up to a possible Smash Bros. public reveal.
The general order of events:
1) Smash Bros. announced for 3DS and Nintendo Wii U
2) Sakurai announces work on Smash Bros. not to start until Kid Icarus Uprising released
3) Kid Icarus Uprising releases.
4) Sakurai states work on Smash Bros. just starting, fans not to expect anything for a while.
5) Sakurai states he enlisted help from Namco Bandai for Smash Bros. 4.
6) Namco Bandai creates teaser site.
Besides the series of events, Namco Bandai does not have any project in the pipeline that matches anything related to "All Star Battle". So we've got three possibilities:
1) Namco Bandai has created a teaser site for Smash Bros. 4
2) Namco Bandai has created a teaser site for their own trademarked All-Star Battle series.
3) Namco Bandai has created a teaser site for a new created project (can't think of the word right now, whatever Mario/Pokemon is to Nintendo, creative license? Is that it?)
#2 is insanely unlikely. By already working on Smash Bros. 4 (confirmed), it would be unwise of them to create a new project to directly compete with their own creation in the gaming market. #3 is possible, but as I stated earlier, this has come at a very pivotal and appropriate time in the Smash Bros. 4 development process to warrant any type of separate Namco Bandai release irrelevant.
EDIT: Not to mention that I have found news articles in at least five different languages (English, Spanish, French, Japanese, and Italian) all speculating Smash Bros..