Not true, many times it is possible to make a grab that will give you an advantage and even reverse the situation. Speed, upper body strength, and very importantly "experience" will work. In Krav Maga we have an exercise where we have to get up from a kneeling position against several attackers. From a mounted position an experienced fighter knows when to defend and escape!<quoted text>
"The fighter on the bottom has no leverage or torque and is rendered nearly incapable of mounting any countering offense. A fighter in this position, unable to throw the fighter on top off balance and unable to escape, is almost certainly doomed to defeat."
...but managed to reach into a jacket pocket, pull out a gun and shoot Martin in the chest?
...doesn't hold water
"...a monkey could try this case, do you hear me, a monkey."
A beginner may be slow to recognize opportunity.