Sure, American Pharoah would have run faster in the Belmont if he had been pushed. That doesn't mean he would have run much better than 2:26 there.
American Pharoah is a very good horse, but Stevens is living in a fantasy world if he thinks any horse could have ever bested Secretariat's record in the Belmont.
He also forgets Secretariat wasn't even pushed, and was running against a headwind in the backstretch, when he set his unassailable Belmont record in 1973. Had Secretariat been pushed at all, he would have knocked another three or four seconds off his 2:24 and won by 100 lengths.
American Pharoah can stand on his own merits. Secretariat is still the greatest racehorse of all time.