I always assumed that we only had to preserve the identity of pprune members. In other words not "out" their true identity. No one has linked a pprune alias to Jo.
Anyway, I have a copy of that email so possibly 4000 was used to justify Flybe getting involved with CTC and to simmer the frustration amongst the applicants. Deano777 obviously knows her personally so maybe 1300 is accurate. It's still a significant number of applicants.