It's also false. Most true xl bullies in the UK often come from imported bloodlines.
In most senses there is no real breed of an xl bully. All the things posted about blood tests are false. The current xl description is based on physical attributes only nothing to do with actual genetics.
Most xl bullies don't come from far back xl bully blood lines, but are often mixes of big breeds such as canecorsos, mastiffs with smaller bull breeds
Hence the fact that quite quickly people were producing micro bullies, exotic bullies or whatever they want to call them because they just changed the mix
It's why the ban on breeding is difficult. There is a ban on xl bully breeding (they have to be neutered) but....
You could breed 2 perfectly legal parents and create a xl (by the legal definition), eg by mixing American bull dogs, corso mixes, staffs with mastiffs, rotties etc
You'll find that some of the dogs that clearly look like xls are maybe 2 cm short of the legal definition so fall outside of the definition. A neighbour close to me had her clearly xl bully seized but then returned as it didnt meet the definition. Someone could legally breed two of those and produce a dog that's easily in the criteria.
The legislation mostly relies on owners following a law. For example all the people that registered there's got slightly hasseled to make sure they were neutered and got checked up on, but all the people who ignored the law in the first place are out there breeding
The problem with anything like having to have registered breeders of all dogs, dog licencing etc is that mostly it's the decent owners who do it, and the dodgy ones continue on doing what they are doing
It's the same with all current dog laws eg not being able to sell pups without seeing their mum, not being able to sell xls, microchipping pups, ear cropping being banned. All 100% the right laws but never followed. The sensible people were doing it anyway, and the people it was aimed at are the People who still ignore it because they aren't bothered about the law. A quick search on pets4homes and you can find hundreds of illegal things happening in plain sight because until there's proper enforcement people aren't bothered.