Here's why it doesn't matter if Warren or her fundraisers say she's not running in 16: she cant possibly know herselfWe can refine this a bit, I suppose. She certainly might know if she's running right now, even though she wouldn't know whether she'll still be running by the Iowa caucuses. And she might want to deny running, even if that's the case. She also might have some reason to be absolutely certain that she would never run, even if the thing dropped in her lap.
But for many candidates at this stage, it's more ambiguous, and the particular structure on the Democratic side in this cycle makes it even more so. Presumably there are several potential candidates who are in if Hillary Clinton drops out, but out if she remains in.
For Clinton, as many of us have noted, it makes sense to delay the overt candidacy as long as possible; not only does that leave all her options open if she's really undecided, but even if she is clearly running for now there's nothing much to be gained by becoming an overt candidate.
The thing is, for Warren or Clinton or whoever, making the move from "doing everything one would need to do to be in a position to being an active candidate in the months leading up to Iowa" to an overt candidacy may very well depend on how the first part of things goes.
Which is to say that for actual politicians going through the process, the decision isn't so much of a "if you could be president would you do it" or even a "if you could be the nominee would you do it" as much as it is "considering the chances of winning both the nomination and the general election, which are constantly shifting, is it worth continuing to move forward?" Which leaves two potentially moving variables: the point at which the politician believes it's worth making the run, and the assessment by the candidate of how close she is to that point. What I mean is that some potential candidate (say, Elizabeth Warren) might definitely be up for being president if it was handed to her and definitely wouldn't run if it was a sure loss without having definitively decided exactly what her perceived chances would have to be for her to be an active candidate in 2016. So deciding whether to run really means figuring out both the odds of winning that make it worth running, and what the current odds of winning might be -- which includes both the overall emerging political context and, as time goes on, what's happening in her own not-quite-a-campaign.
Combine all that with some legal and some practical political reasons for candidates to avoid overt candidacies until fairly late in the process -- along with a system in which contesting the nomination actually begins (at least) three years before Iowa. So we have good reason to heavily discount whatever they say about their (current? possible?) candidacies.
Therefore, in real life, we don't really have two separate piles of candidates and non-candidates; we get a whole bunch of people who are in between. Especially in the first two years of the cycle.
In other words? A very good shorthand for all of that is "she can't possibly know herself." So: nice catch!