Nothing new here
Read "Influence: The Psychology of Persuasion" by Robert Cialdini. It's an excellent and very easy to read book about non-obvious issues in psychology. I don't have my copy to hand, but one of the chapters discusses how writing down or publicly stating an opinion can make you more committed to it.
One example of this discussed in the book was during (I think) the Korean war, American prisoners were offered small rewards (such as an extra portion of rice) for writing anti-American essays. Prisoners were happy to write what they considered to be anti-American lies to get the rewards because they figured that everybody would realize the lies were written under duress. However, the act of writing the lies actually started to subtly change the viewpoints of the prisoners and many of them did form anti-American viewpoints over time.
Another example concerns the competitions run by many companies: complete the following sentence in 10 words or less "I love <name-of-product> because...". The goal of these competitions is to target lots of people who think "I don't like this product but I'll write something positive about it to have a chance of winning". Just the act of writing something positive about the product will cause some of those people to start liking the product.
By the way, the change in attitude brought about by writing a message can be brought about in other ways too, such as standing up in a roomful of people and voicing a message that you don't actually believe. The important thing is that you carry out an act (such as writing or speaking) that apparently shows commitment to the message *even* *if* you don't actually believe in the message that you are communicating. Such acts can change your viewpoint.
So, back to this report on the texting and obesity study. By texting a message of your progress, the person is carrying out an act of commitment. This makes the person more committed to achieve the weight-loss goal. The "Influence" book discusses how Weight Watchers uses a similar tactic to reinforce commitment of its members. So this scientific study appears to have been carried out by people who were unaware of prior work in this area. The researchers haven't found anything new; they have just confirmed something that was already known.