It's well known that the judges watch the rehearsals. They've never hidden this & several times in the past they have made comments relating to this e.g. 'that was not as good as the rehearsal' or 'it was good to see you pulled off x move as it went wrong in rehearsal' etc.
Jason's comment was obviously scripted based on the rehearsal but a decision was taken to take the lift out of Luke's performance & he just read off his script rather than having the common sense to edit his comments having seen the main performance
But wasn't the VT from before the day of the performance/final rehearsal - it didn't look like the studio? And why would they do a dress rehearsal somewhere else?
And if he had seen a VT in the studio as natalian suggests they can, was it a different VT to the one we saw which would have told him about Luke not doing it?