Oda writing a character saying something is not the same as Oda confirming something. Otherwise, every instance of hyperbole, like Enel saying "My will alone created the universe" or non-truth, like the World Government saying Chopper is a pet of the Strawhats, would be canon fact, and neither of those things are despite Oda writing a character saying them.
For a character's words to be taken as confirmation from Oda, it has to be credible. Urouge is not. He is an outsider of the crew who has never met them and knows a whopping total of nothing about the crew. Him saying Zoro is the first mate is as credible as the World Government saying Chopper is a pet. Unless it comes from Luffy and the crew, it's not confirmation, and the only thing to come from Luffy is a throwaway line about Usopp taking over.
The Zoro-Kuma situation is so overblown, considering that Kuma told the residents of Thriller Bark he'd kill all of them if they don't hand over Luffy, and all of them chose to die, which is the exact same thing Zoro did with the pain pubble. It's not special.
That goes double for Zoro yelling at Luffy, since Luffy continued on to be goofy immediately after(much to Zoro's ire) and other Strawhats, including Nami, Franky, and Sanji, have yelled at him or stopped him from being stupid/wild at one point or another in the same fashion.
