According to the article, the trial will only include a fraction of people who would be eligible for this treatment. I interpret that as meaning they will be ring-fencing people who are eligible and off work, and monitoring whether they a. lose weight and b. get back into the workplace.
There are plenty of people who are eligible but who are managing to work despite their health conditions, who won't be included.
Presumably people on the trial will know the aim of it is to get them back into work, and will be happy with this aim, so the results will be biased in favour of success,
People who are happy not working won't be on the trial, so money isn't going to be wasted on this method of getting them into the workplace. A future outcome might be that, if the trial is successful, people whose reason for being off work is obesity-related might be forced to accept treatment or lose benefits, but I doubt that because there will always be reasons people can give for not accepting treatment - they'd only have to say they were suffering side-effects (real or not) and they'd be taken off it.