I can counter argue it with this:
Sanji keeps the mood of the crew and motivates them if it's necessary, just like a real leader.
Sanji keeps Luffy under control
Sanji organizes the crew, has a plan and leads the crew when Luffy and Zoro fu"k around.
Sanji again keeps the mood of the crew and explains to Jinbe why they got to fight.
Sanji does the same in Alabasta
Sanji confronts Law realizing that he has an agenda.
Sanji again keeps the mood of the crew and explains Choper the situation.
In absence of Captain Sanji takes control of the crew and and makes calm decisions when Nami and Brook were in panicking because of Big Mom's ship.
Sanji again argonizes the crew, he has a plan and leads the crew and even Franky who wasn't a crew member back then excepts Sanji's authority.
Sanji understands the situation very well and gives Choper specific orders and he also has a plan as always.
And little bit more of Sanji's leadership