It's a bit of a leap from the kid fibbing about where he sat during takeoff, to a full-blown conspiracy theory involving the press. After being found out, the little toerag is going to say anything that he hopes will minimise the trouble he's in.
As I said in a previous post, Jet2 pre-assigns seating, unlike easyJet and Ryanair. He just couldn't get on and sit in a free seat, confident it hadn't been pre-assigned to someone else.
No he couldn't be sure, but depending on how many pax were on the flight (do we know?), his chances of picking an unassigned seat might well have been better than odds-on, and of course the later he boarded, the greater the chance that any unoccupied seats would remain so.
I may be proven wrong, but I don't buy active cooperation of any third party.