1 in a million. Unless the dev was a moron.
Not necessarily... I don't want to get into another tedious dissection of the word choice of the original Eurogamer rumour, but it's not clear to me that "official documents" means some sort of extremely revelatory document. An official document here might just have been a minor internal memo written on company letterhead, with some sort of oblique reference to the development of ESV. I don't think it's so implausible that a developer would carry those sorts of documents on a plane, if she/he was travelling for business, or wanted to read over some business correspondence, or whatever. Leaking them is another matter, of course.
Also, just for further [censored] and giggles, it need not be the case that the "direct sequel" remark comes from those "official documents". Recall that the rumour states: "The same source confirmed, with official game documents in hand, that this will be the chronological(/direct) sequel to what happened in The Elder Scrolls IV: Oblivion". This is consistent with the source telling the Eurogamer dude that ESV is a "direct sequel", and the documents containing no information of the sort. But this might not matter so much - if the rumour is to be believed (and I doubt that it is), then the "direct sequel" remark would seem to have equal credibility whether or not it was contained in those "official documents".