Serebii doesn't claim who its source is, but given his past behavior, I'm willing to bet he's just trying to appear on an even key with Pokebeach. Like I showed earlier, this happened before with the falsely leaked Shaymin, where Pokebeach and Serebii kept trying to one up each other, making it to where they claimed they had a source.
Pokebeach says he was won over by the spanish website's description of the pokemon covered in junk.
However other than that description he has nothing to go off of, and little reason to believe him. Secondly, the DAY they were released to 2chan the spanish websites reports them as confirmed. There have been NO other reports of insider information before this. Then when you consider the incredibly fake image posted from the webhost via his twitter, the validity of his claims are incredibly questionable, not to mention that the pokemon he allegedly describes to Pokebeach, he was unable to descern the uniform that was clearly a separate piece from the pokemon as being apart of him.