That was what was printed at the time - that Nigel stole all the data on a CD or jump drive. Then he made a copy of it for Mike, who at the time was working for Alonso. He used that data to set up Alonso's car without telling anybody where he was getting his ideas from. When it started showing dividends, then he and Nigel went to Honda for the job interview talking about how they each had "nurtured" this information and that it worked and they were wanting jobs.
It all sounded a bit too good to be true to Honda, so they notified the FIA who then quietly called Ferrari to ask if they had anything "missing". I believe Honda actually supplied a small bit of the data which Ferrari immediately recognized and the whole thing blew sky high from there.
The one thing for sure though is that Ron Dennis, nor anybody else at McLaren except for Mike and Alonso ever knew about the whole damn thing, and it was Ron and McLaren that wound up paying the tab for it. All of it.