Who's theory is this? Every vet I've talked to say that IBD is strictly contagious to fluids passing to and fro other snakes. Which means- Mites, feces, or sex.
Regarding playing Russian roulette with your collection - per the Merck Veterinary Manual:
Considered is the keyword there. It's also been considered that IBD originated from Green Burms too. That it was passed to and fro with Boas being housed with Burms back in the day.
I'm guessing the only reason a test is done is because something is wrong, right? I'm not going to sacrifice a completely healthy snake to be tested. Would you? Thus, 100% of those tested had problems to begin with, showing signs of IBD before death.
Only adult boas are known to carry IBD for long periods of time. Baby Boas will die as quickly as Ball Pythons (or other snakes) when introduced to it. This is the main way people learn they have it in their collection. Their baby boas die off.
Can you name any? I know of 2 people who've had IBD in their collection right off hand. One of those is heresay, the other (BWSmith) was very public about his problems and still has a few babies he had during this whole dilemma and is most probably the most educated person about this topic I know. He had live biopsies done on his whole collection and had to put down the majority of that collection. He still has a few remaining boas that were at his house during his breakout.