Short answer: No, but you should.
Long answer: The point of the liquid preop diet is to make both your body and psyche adapt for what comes next. Liquid protein supplements are the ideal form, since you do not lose muscle mass and accomplish your goal, basically a win-win. It also makes surgery easier in case where you have conditions like fatty liver that may obstruct the surgeons view, and believe me that having solids in your body will literally be a pain when your intestinal transit goes back to normal and you cannot expel them properly due to not being able to apply the proper abdominal pressure.
When it comes to the bottom line, your body won't implode or anything, but it will make things easier for both the surgeon and yourself.