>>713805107
He just said he had a serene heart and got shifted by rage. Krillin was the one who said you needed a serene heart to become a Super Saiyan, much later, and this one you might attribute to an instance of "author's knowledge gets shared with the characters", or you could think of ways that would explain it. Toriyama often has his characters operate on incomplete information and make misguided assumptions, so that would be a bit of an outlier.