“There is growing evidence across Europe that terrorist prisoners are successfully deceiving professionals concerning their rehabilitation."
that's not a new thing and it isn't exclusive to "terrorist prisoners" - a very large percentage (if not a majority) of habitual, recidivist criminals often "find Jesus" - or some other religion while incarcerated.
Even people of average and below average IQ are adept at intuitively knowing what authority figures want to hear when tested, examined, interviewed, screened for "early parole hearings and such"
Small children seem to instinctively know what things they say or do will alternatively please and displease their parents.
I believe this is dealing with jihadi recidivism. The incidence of rehabilitation is very small because of the intensity of impact the ideology has on the individual.