RIP the deceased Gnat pilot...
As for 'aviation expert Julian Bray', whose cellphone number is easily found by Googling, he is talking complete and utter nonsense.
Elsewhere it has been written that
...from the video it would appear that it was a classic flick and spin .. pulling too hard too slow...
From what I recall from flying the Pocket Rocket, 'pulling too hard too slow' would lead to heavy buffet and a high RoD. But not 'a classic flick and spin'.
The Gnat is a tricky little jet when things go wrong, purely due to the complexity of its systems. STUPRECCC and CUBSTUNT drills have to be instinctive, but it does NOT have any unpleasant aerodynamic quirks. Unlike the Hawk, it has classic swept-wing characteristics and needs to be respected for such.
Again, RIP the deceased pilot and condolences to family and friends.