It’s probably going to be a legit power
So far when Garp used it, it did just the same as haki. Coincidentally, Garp has only been known to train his body and haki.
Then there's the question: How would Sanji learn about this power and its name? When did he acquire it, too? Why wasn't it relevant when other characters, himself, and even the narrator mentioned Sanji's abilities again? Why were only haki and his exoskeleton mentioned as sources of his powers when he used Ifrit?
Nah, it's not its own thing.