Looking at the judgement, the Crown did argue that the witnesses could have been fed the information, but this wasn't enough to convince the Appeal judges not to allow a retrial - basically they refer the question to the jury.
"Nevertheless, we have borne very much in mind the fact that there may be material available to the Crown that has the potential to undermine the witnesses' credibility at a hearing where their evidence can be thoroughly and rigorously tested in a way that was not possible before us. The Crown has a number of potentially valid points to make. There is the possibility that the witnesses may be more partial than they cared to admit and there is the obvious point that their current accounts did not come to light until the appellant had exhausted the appeals process."
The Crown also argued that the sexual behaviour was pretty standard. However:
"The flaw in Ms Law's argument, as we see it, is that she focussed on there being nothing 'unusual' about X's alleged behaviour. The behaviour does not have to be unusual or bizarre; it has to be sufficiently similar that it cannot be explained reasonably as a coincidence".
It just seems so flimsy, given the potential for the information to be fed to the witnesses, and, yes, the pretty common nature of the behaviour. However, apparently it was enough to convince the jury of reasonable doubt.
What isn't clear is why this case is 'exceptional' and why a similar tactic couldn't be used by another defendant. The main exceptional thing about this case seems to be the amount of resources the defendent had at their disposal to keep going until they found anything to advance their case.