It usually helps, but as with most obesity-linked conditions it's not suitable on its own as a first-line treatment. The effect is small compared to CPAP, and sufficient weight loss to see that effect takes months, whereas CPAP starts working literally overnight (DME/insurance shenanigans notwithstanding).

Also, chronic sleep disruption is associated with a cluster of symptoms (impaired insulin sensitivity, lethargy, increased hunger) that make it very difficult to lose weight in a normal/healthy way. Many patients find that they lose 10-20 pounds after starting CPAP with no conscious change in their lifestyle.