Training wasn't really the cause of Sanji unlocking the exoskeleton.
It was his kindness and compassion to protect weaker people that made Sanji use the RS, which Sanji's pride didn't allow him to use, but Sanji's empathy in protecting his crew and Momo made him use the RS to unlock the exoskeleton later on.
After all, Sanji is Sanji and Zoro is Zoro.
Zoro gets stronger with training, Sanji gets stronger throughout the journey always with the intention of protecting the weaker crew members, that's why Sanji is portrayed as the team's protective guardian saving them at all times, he gets stronger progressively to protect the weaker.