yes, the description’s 4-6 feet seems optimistic...
But for the reference: NATO spec. 9mm 124 FMJ can penetrate in excess of 32 inches of calibrated ordnance gelatin as a “general rule”.
Buffalo Bore is pretty meticulous in their claims. For example, they do not list ridiculous velocities from test barrels like most manufacturers. They list actual chrono results from specific pistols of varying barrel lengths.
IE.. they tell you from a 1911, a glock 19, and a subcompact of some kind.
If they say 4 to 6 feet, you can take it to the bank. It HAS been proven. And the variance probably depends on how much bone it has to break through.