The main purpose fo the pre-op diet is not weight loss. It is to shrink your liver to make surgery safer. Weight loss isn't the primary concern.
Once you are postoperative things are much different. The surgery isn't primarily about food restriction, although that is certainly part of it. Having the surgery resets many hormone signals to your brain and other parts of your body. During pre-surgery, your body's metabolism will slow down during the pre-op diet because it sees the reduction in calories as starvation. After surgery, your body will actually increase metabolism because it has been rewired to lose weight.
You shouldn't worry about not losing weight prior to surgery, but you should follow the pre-op diet to the letter. It will make your surgery safer and easier for you and the surgeon.