This tip is more of a suggestion for comfort. If someone eats a big meal before laying on a table for a half hour+, it is likely they may have that "massively full" feeling which can distract from being able to relax. Also, as the body attempts to digest that big meal, it can cause the stomach to make noises, and there may be some gassy or bloating discomfort, and that all can distract from the relaxation and other benefits of the massage.
Alcohol presents other issues. Legally, massage therapists can not work on anyone who is intoxicated because before a massage, someone must give consent to receive the massage, and if they are not in their right mind, and therefore can not give informed consent and the therapist can be arrested for assault. Also, if someone has alcohol in their system, their senses can be dulled, so they may not be able to tell or communicate if pressure is too deep, or if a stretch is done too far, which can result in the massage doing more harm than good.