There is, as usual, a tiny grain of truth behind the myth..
Though the nanny dog thing has always been total bullshit...
Dogs bred to fight in pits in organised fighting, fight several rounds or matches, much like boxers do.
Unlike boxers they don't respect a bell at the end of each bout.
So each dog has a handler (and this may not be someone the dog knows), who jumps in to break and re-match.
To do this, the dogs MUST tolerate the proximity and handling, fairly roughly, by humans who may be strangers do them, whilst they are scared, in pain, on an adrenaline high, hugely aroused....
These dogs must also tolerate having wounds dressed and treated without pain relief or sedation (sometimes that is available but not guaranteed and of course these dogs do not just go to the vets!)...
There was, in the thankfully fairly distant days of dog fighting, absolutely NO place for a dog that was human aggressive - and so those dogs that were did not last, they were killed, they were not bred from, and they did not pass on their genes.
Only those who did well in fights, which meant did well at being tolerant to humans whilst at their absolute worst, would breed.
So as dangerous as a pitbull can be - they were not people aggressive particularly often and whilst you get the odd rogue case, there'd be some pretty clear mitigating circumstances.
The XL bully is a totally different beast - they are NOT bred to tolerate rough handling or selectively to rule out human aggression. Some are bred FOR human aggression, for a willingness to bite and hang on, bring down and dissect.
And for dog aggression.
Of course the XL bully in the UK is a mongrel - and some may have long inbred ancestries with multiple human aggressive animals in the pedigree, on purpose...
And some may be little more than a lab/pit/rottie cross with very little human aggression in there and lots of 'tolerant to human' genetics.
We don't know, we can't know, we have zero way of telling which genetics these dogs have.
Add on top of that, shit handling, housing, training, pain, stress and so on, all of which increases the chances of a dog suppressing body language and reacting 'without warning', increases the chances of an aggressive response, of aggression towards handlers...
You get to where we have gotten now - total chaos.