Sir Isaac Newton has a formula that disagrees. F=MA.
I'm not sure I would recommend a Glock 9mm for a novice for self-defense. I think more along the lines of a hammerless .38 special. Point and pull the trigger until it goes click.
It’s likely that someone said something to the effect of “It’s better to hit someone with a 9mm than to miss with a .40 or .45. A 9mm will kill you just as well.” Then she ran with it.
My .40 has considerable muzzle flip, while my .45 stays on target much easier. I have nothing against 9mm, but I prefer the .45 - and I’m certainly not going to volunteer to stand in front of anything, whether it’s .17 or .50.
I suppose if it is a sold strike between the eyes of the perp, it is basically equal in effectiveness, but otherwise, make mine a .45 ACP, 'cause shooting twice is just silly!
Here is a link to a video of 9mm vs .45acp in both FMJ and Hollow Point and their effect on a watermelon. This is why I carry a .45 and have for 40 years.
LLS
“Anybody else disagree with this statement?”
As with a lot of things in life, the best answer is often “that depends.” Chuck Hawks has written extensively on the stopping power of various calibers, and his stats seem to support the proposition that high quality 9mm defensive rounds have nearly if not the same stopping power as .40 cal and .45 acp. There are exceptions to just about everything, but in general, 9mm +p hollow points are as effective as the larger rounds.
http://www.chuckhawks.com/handgun_power_chart.htm
http://www.chuckhawks.com/handgun_stopping_power.htm
http://www.chuckhawks.com/beginners_stopping_power.htm