This claim is misleading. The article from Marathon Handbook included in this claim states that it is "technically possible" for healthy adults to finish a marathon without training, but is not safe or recommended. A healthy adult may be able to walk a marathon or even jog a six or seven hour marathon with little to no training, but in terms of actually running and finishing an hours long marathon, it can lead to high risk of injury, muscle soreness, and overall disappointment, "if not worse." The article emphasizes, "We do not endorse running a marathon without proper training."
The article also makes sure to cover the risks of running a marathon not having received adequate training, including: increased risk of myocardial fibrosis (the buildup of scar tissue in the heart muscle which can make it harder for the heart to pump blood effectively), increased risk of running-induced injuries such as knee pain and stress fractures, stress on the kidneys, damage to DNA and muscle tissue (red cell breakdown, skeletal muscle cell damage, increased cortisol, blood in the urine, etc.), and mental burnout. Since most experienced marathon runners train for 3-6 months leading up to a marathon, those who have no experience are more prone to these health risks. The author implores beginners to "respect the marathon distance and go through a proper training plan." (Marathon Handbook).
OCMarathon, the official site for the Hoag Orange County Marathon Running Festival, also states the risk of running a marathon unprepared, highlighting risk of muscle strain, dehydration, exhaustion, and injury. The article however, also includes benefits for running a marathon with proper training: improved endurance, improved strength, and improved fitness. (OCMarathon).