One game does not a season make.
Tua and Trevor completed 69.0 % and 65.2 % of their passes respectively.
Their YPA’s were 11.2 and 8.3 respectively, so the ‘season’s stats don’t support your claim
Yards per attempt don't tell the story of your downfield, completes passes. Yards PER COMPLETION do.
Below are Tu'a's completions of 20 or more yards in the 2018 season. Now consider that he was out in the third quarter of a lot of his games and as soon as the 2nd quarter of some as well. Jalen Hurts added another dozen or so downfield throws while he was in the game, but here are Tu'a's...
DeVonta Smith: 28 yards
Devonta Smith: 26 yards
Jaylen Waddle: 49 yards, touchdown
DeVonta Smith: 27 yards
Jerry Jeudy: 25 yards
Irv Smith: 32 yards
Jerry Jeudy: 58 yards, touchdown
Henry Ruggs III: 31 yards, touchdown
Irv Smith: 22 yards
Devonta Smith: 41 yards, touchdown
Henry Ruggs III: 20 yards
DeVonta Smith: 30 yards
Jerry Jeudy: 79 yards, touchdown
Jerry Jeudy: 35 yards
Jerry Jeudy: 22 yards, touchdown
Jaylen Waddle: 30 yards
DeVonta Smith: 30 yards, touchdown
Josh Jacobs: 25 yards
Jerry Jeudy: 20 yards
Irv Smith: 21 yards
Hale Hentges: 23 yards, touchdown
Damien Harris: 52 yards
Jerry Jeudy: 21 yards
Irv Smith: 42 yards
Henry Ruggs III: 57 yards
Jerry Jeudy: 34 yards
Jerry Jeudy: 24 yards
Irv Smith: 47 yards
Henry Ruggs: 21 yards
Jerry Jeudy: 42 yards, touchdown
Jerry Jeudy: 60 yards, touchdown
Jerry Jeudy: 22 yards
DeVonta Smith: 21 yards
DeVonta Smith: 57 yards
Damien Harris: 20 yards
Jaylen Waddle: 77 yards, touchdown
Jerry Jeudy: 31 yards
Jaylen Waddle: 22 yards
Irv Smith: 25 yards
Henry Ruggs III: 45 yards, touchdown
Jerry Jeudy: 29 yards
Irv Smith: 25 yards, touchdown
Jerry Jeudy: 30 yards
Jerry Jeudy: 24 yards
Irv Smith: 25 yards
Irv Smith: 21 yards
Jaylen Waddle: 29 yards
Jaylen Waddle: 21 yards, touchdown
Jerry Jeudy: 22 yards
Henry Ruggs III: 22 yards
Henry Ruggs III: 54 yards
Irv Smith: 68 yards, touchdown
Henry Ruggs: 21 yards
Jerry Jeudy: 46 yards, touchdown
Josh Jacobs: 33 yards, touchdown
DeVonta Smith: 20 yards
DeVonta Smith: 40 yards, touchdown
Henry Ruggs III: 22 yards
Jaylen Waddle: 53 yards, touchdown
Jaylen Waddle: 23 yards
Jaylen Waddle: 51 yards, touchdown
DeVonta Smith: 50 yards
Jerry Jeudy: 40 yards
Josh Jacobs: 27 yards, touchdown
DeVonta Smith: 21 yards
Jerry Jeudy: 62 yards, touchdown
Irv Smith: 21 yards
DeVonta Smith: 23 yards
Jerry Jeudy: 48 yards
73 completions that Tu'a completed for 20 or more yards. 22 of those passes went for touchdowns. 21 of those passes went for 40 or more yards
Now here are Ian Book's completions for 20 or more yards...
Alize Mack: 24 yards
Michael Young: 66 yards
Tony Jones: 30 yards
Chase Claypool: 35 yards
Kevin Austin: 29 yards
Miles Boykin: 33 yards
Miles Boykin: 20 yards
Ian Book: 35 yards, touchdown
Chris Finke: 56 yards
Miles Boykin: 40 yards, touchdown
Miles Boykin: 20 yards
Miles Boykin: 30 yards
Chase Claypool: 21 yards
Miles Boykin: 35 yards, touchdown
Kevin Austin: 28 yards
Jafar Armstrong: 25 yards
Cole Kmet: 24 yards
Chris Finke: 26 yards
Jafar Armstrong: 27 yards
Miles Boykin: 22 yards, touchdown
Cole Kmet: 24 yards
Miles Boykin: 20 yards, touchdown
Chase Claypool: 31 yards
Chris Finke: 26 yards
Chase Claypool: 21 yards
Chase Claypool: 27 yards
Chase Claypool: 33 yards
Ian Book: 47 yards
Miles Boykin: 24 yards
Chris Finke: 24 yards, touchdown
Chase Claypool: 20 yards
Miles Boykin: 38 yards
Miles Boykin: 23 yards
Chase Claypool: 28 yards
Dexter Williams: 22 yards
Tony Jones: 51 yards, touchdown
36 completions that Book completed for 20 or more yards. 7 of those passes went for touchdowns. 4 of those passes went for 40 or more yards.
Tu'a Tagavailoa: 255 / 355 attempts, 73 completions of 20 yards or more. 21 completions of 40 yards or more. 22 touchdowns of 20 yards or greater.
Ian Book: 214/314 attempts, 36 completions of 20 yards or more. 4 completions of 40 yards or more. 7 touchdowns of 20 yards or greater.
On a similar number of attempts, Tu'a far out performed Book as a downfield passer. Both in terms of yardage and touchdown totals. Number of games played is irrelevant, only pass attempts matter and in that case, both players had a similar number attempted. There were also a lot of games that Alabama was crushing their opponent and Saban refused to throw deep passes... And I do realize that some of the above plays for both guys were shorter throws that their receivers turned into longer plays, but overwhelmingly, both players' chunk plays through the air came on downfield throws. I'm not about to watch every snap to pick out the few passes by each guy that were short passes that wound up going for big yardage.