Regarding Seabiscuit and Whirlaway.
Seabiscuit was every bit the handicap champion that Whirlaway was and probably a little bit more. He was also more tactically versatile than Whirlaway and likely would have beaten him in a match race.
Here is a cut of Seabiscuit as an older horse:
Note the difference in weights that he's carrying against his top finishing rival.
Three times in a row, he was 2nd by a neck. In those three races, the horse who beat him got a weight break of 30 pounds, 12 pounds, and 15 pounds.
Even in all of his wins, Seabiscuit was carrying bowling balls more than his opposition. He was facing top competition, racing in big fields, and traveling all over the country.
That said, Whirlaway is clearly the higher rated overall horse. Seabiscuit was mediocre until very late into his 3yo season when he exploded in Northern California. I don't think Seabiscuit's win over War Admiral was any kind of fluke. He looked the better horse and War Admiral didn't run to his best form at Pimlico. He won the Preakness in a desperate battle as a 3yo and made hard work of weak competition in the prior years Pimlico Special. War Admiral bounced right back and won his next race. He had no excuse in the match race.
Seabiscuit has a suspicious form and his trainer later came under suspicion.