I don't think this is entirely true. It was a different time back then, there really weren't any top European gyms that Cro Cop could have trained at. Plus, he trained in Croatia. You can count on one hand the amount of top Croatian fighters there are in MMA history. And he was the first all-time great one, so really not much he could have done unless he left Croatia to train somewhere else. Not sure how that makes him arrogant, hindsight's 20/20, every fighter would be better if they went to a top gym.
You're also probably misinterpreting the context of the Drvis thing, pretty sure he's one of Cro Cop's best friends and he was not being entirely serious lol, sure he may have said it in a serious tone but... yeah.
He came into MMA at a weird time where everybody was still sort of learning on the spot. He didn't train in a cage before he got into the UFC, then he's like oh shit I probably need to start training in a cage because this shit is different. These are things most fighters today don't have to worry about.
And the last line doesn't really make sense because he came into MMA as a pure kickboxer, literally straight out of K1, learning on the spot more often than not... I'm not sure how you couldn't see tons of improvements when it comes to his overall striking arsenal and other aspects of MMA over the years.
EDIT: It's Cro Cop's birthday today. Happy birthday to the legend, gets way too much disrespect on this forum.