It has been stated much earlier in the series that Zoro keeps Luffy above everything, so much so that he's willing to die and thus, abandon his own ambition for him. He has literally said, "How can I protect my ambition when I can't even protect my captain?!".
At first, Luffy was a side quest to his main quest and who Luffy is as a person didn't really matter to him as far as morals go.
After that Zoro loses to Mihawk who tells him Luffy isn't a side quest to Zoro. He tells Zoro that he must in fact go see the world before he can hope to be ready and so Zoro has to make the choice to change the meaning of his dream to the man who will be the strongest swordsman in order to make Luffy king of the pirates. That was a very significant Luffy-Zoro moment.
Zoro now has to start caring about Luffy's adventure and who luffy is since he has to travel with luffy around the world to get the experience required to beat Mihawk. In fact, after this is when Zoro starts adopting some of the luffy mentalities and morals such as how he dealt with the Nami Ussop business in Arlong park.
Dying for your friends is different than dying for their ambition.
Protecting the crew at thriller barl fits perfectly with zoro's swordsmenship, "to protect what one wants to protect" zoro succeded in that. It was his swordsmanship that saved his crew.
Nothing post TS proves that zoro places luffy's ambition above his own, yes he trust luffy to achieve his dream but luffy too trusts zoro to do the same they were working together to achieve their dreams.
Also putting your life and ambition on the line to save a friend doesn't mean you place their ambition above yours, otherwise luffy considers robin and sanji's ambition more important than becoming the PK since he too sacrificed his life for them once.
Zoro swalowed his pride to ask mihawk to train him following luffy's decesion of taking a break and getting stronger for the new world, luffy too swallowed his pride and bowed two times to save his freinds the 1st at drum kingdom looking for a doctor for nami and the 2nd at amazon lily.
Life and ambition worth nothing when it comrs to saving a friend's life, that's why whitebeard pirates sacrificed hundreds of comrades just to save ace,
it was never because of hierarchy, but rather friendship.