Nothing to it. If speed brakes won’t slow you down enough to join the formation, a barrel roll over the top will. I think the pilot deserves kudos.
I think the real issue was his being late for a unified formation, and out of uniform formation.
He either was disorganized, late or did something out of line which caused him have to resort to the move because he was carrying much more speed than the formation he was supposed to join.
The air force frowns on pilots who fail to be part of a tight unit. It shows a lack of discipline and professionalism.
I can see why the Wing Commander had the lad placed on review.