Of course you understand that this is David Rigert, former world champion olympic lifter who has lifted (among others):
660lbs x 6 reps on back squat
880lbs deadlift from blocks (below knees)
440lbs POWER clean during his competitive years and later 485lbs at a higher bodyweight
352lbs POWER snatch
485lbs clean & jerk
380lbs snatch (later 440 at a higher body weight)
That would be in large part responsible for his spinal erectors. Of course you can do isolation work to build them up, but if you deadlift and squat 400 and clean 200 I doubt that you can reach anywhere close to that level.