It is true, however, it doesn't explain why "pits" were quite popular pet dogs a CENTURY ago - when they were closer to the actual evils of deliberate fighting.
I'm not a pit fan, but the reason they can be so deadly now is because they are popular with fighting rings; they breed them for certain traits, give away or sell some of the puppies, they in turn are bred, and so on. In the eighties it was Dobermans. Toy poodles, apple head chihuahuas and yorkies are the same way; since people (puppy mills) breed for type, and not temperment, most of them are vicious little rat dogs. Not mine, though. Mine's a little pink angel. ;-P
The explanation I've heard and believe is that pits as fighting dogs were bred to be easily handled by their human owners who needed to manage them in order to compete them.
People aggressive and dog aggressive are two different things. Terriers are all naturally dog aggressive. At conformation shows they stand terriers face to face to see them respond aggressively as part of the standard. I just saw on TV this Jack Russell competition chasing bait down a hole and they were all muzzled. Some would stop mid chase and attack the dog next to it muzzle and all. It was disgusting. Some sport.
The difference between today and a century ago is that today I would guess the macho criminal element breeds pits for people aggression. Pits were never meant as guard dogs and have no judgement for it. Unlike GS...