We never really got a proper explanation but considering that RS forced Sanji to unlock his exoskeleton, i think it's safe to assume the DNA was already in the process of awakening since he was child, but was simply gradually manifesting with his growth
Queen stated his diable jambe power must come from the modification(since he is not a lunarian) because he is a scientist himself. Sanji on the other hand believed to be a normal human, but like this spoiler suggest, he is wondering what is happening to his body simply because he doesn't know it himself
The RS being created only to fit for the original stealthblack through DNA most likely just increased the process of fully awakening his exoskeleton through his DNA modification
That's why i think Oda is going the road that Sanji's DNA modification simply was just a late bloomer, with it's awakening expanding with his growth and the Raid Suit simply increasing that process