Interesting. Note: I can't go read the original paper as it's behind login, but based on my guess at what kind of goals they're talking about it's mainly aimed at "identity"-based commitments (though a long-range goal is also mentioned).

eg claiming "I'm a writer / I'm writing a novel" as opposed to "I will write 50,000 words this month"

I speculate that there is a big difference between the two above claims. The first one, you can get away with claiming - and gain social credit for not doing anything to accomplish it apart from carrying a writing journal and... I dunno, dressing bohemian and living in a garret for a while. You can "be writing a novel" for years...

But if you make a very specific claim eg "50,000 words written in November" - a claim that can be backed-up by evidence of regular progress (for nanowrimo... anybody can go see your actual word-count), then I reckon you can't back down quite so easily.