That article is about academics and has little relevance when it comes to martial arts. This issue is quite simple, kaido has been undoubtedly yamatos strongest opponent and sparing partner, there is a bit of a zolo parallel here as well if you think about how zolo trained under mihawk and learned from him while staying committed to beating him. Also this is just an assumption but I would argue that kaido wanting his child to be strong was actually in his own twisted way training Yamato even if it was to an extreme level where failure or weakness could mean certain death.
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I agree, lots of people thought wano was supposed to be zoro's character arc with more backstory. I hope we at least get to see him visit ryumas grave and learn something about the shimotsuki after greenbull and before they leave wano