I keep hearing that, but I don't remember anything in game supporting it. Was it stated in a comic somewhere or something?
Basically from both events in the boss battle and dialogue from Kessler.
At the beginning of their fight, after Cole barely managed to chip off like a quarter of Kessler's health bar, a quick time even triggers where Kessler ragdolls Cole with his TK, they get into a grapple, but Kessler actually easily overwhelms Cole in a contest of strength, and zaps him with electricity. All while stating his disappointment in Cole's weakness and claiming that the whole experiment in Empire City was a failure (since the whole goal of it was to make Cole stronger than Kessler, which wasn't the case at the time). Just as he's about to finish Cole off, Zeke swoops in to save the day, until Kessler sends him flying with a casual shockwave.
Cole gets up, more pissed off than ever and gets back into the fight. But then Kessler notes how there was gonna be no more messing around and starts to bring out more and more of his powers to show how serious he is. Throughout he whole fight, you notice how Kessler has "stages" according to how low his health is, in each stage he reveals new abilities so that he can take Cole down, accomplish dialogue along the lines of "its gonna get worse" or something staying that he's getting more serious.
And then at the end of the fight, Cole actually manages to overpower Kessler in a contest of strength and even send him flying with an uppercut, even though at the beginning of the fight Kessler easily overpowered him. Though as we saw from the fight, Kessler did have a stamina problem likely due to his old age, as every time he fired off 3-4 lighting beams, he'd drop to his knees in exhaustion
TLDR; Kessler was initially kicking Cole's ass while not even using all of his powers, but Cole still managed to defeat him after getting pissed off enough and using everything he had