Brock wasn't great at football because he wasn't a football player, competing with guys who had played thir whole lives. Not because he wasn't as athletic as they were. Let's compare Brock, who nearly walked into the NFL after a year or so of solo training, and JJ Watt, who entered the NFL fresh out of D1 football, and is considered among the most athletic in the NFL...
Brock:
Weight - 283lbs
40 yd - 4.65
Broad Jump - 10ft
Vert - 35"
235 Bench - 30 reps
Watt:
Weight - 290lbs
40 yd - 4.84
Broad Jump - 10ft
Vert - 37"
235 Bench - 34 reps
So apparently, one of the best defensive players of this era, is about as athletic as an NFL reject, who took up the sport on a whim, in all of the areas generally recognized as measures of athleticism by the NFL.