However, the basic laws of physics clearly show otherwise . . . And note the caveat that your excerpt includes: “in proportion to their jaw size”, which is in fact much larger than most other breeds, being both broader and thicker-boned. And the claim that their “jaw structure is no different from any other breed” is plainly preposterous — collies and English bulldogs are similar size dogs overall, but both have wildly different jaw structure from a pit bull.
Leverage is a significant factor — a long-jawed dog like a Collie can bite down with all it’s might, but the harder it bites, the more opposing force is tranferred back to the joint and muscles controlling the jaw closing, because of the long distance between the point where the object is gripped and the source of the dog’s pressure. And the narrower jaws that most breeds have limits how large a chunk of flesh/muscle they can grip firmly at once. And very short-jawed dogs like boxers also can’t get much flesh/muscle into their jaw-grip at once, even though they can clamp down very hard on whatever do they manage to get in. The functional morphology of different dog breeds’ jaws is very, very different, even within the same general size of dog or jaw.