Other people do it too. It's the same as people shitting out body hardening while being normal. This is a manga and people have special powers. There is literally no reason to believe soras drug somehow altered sanji. And again, just being a zoan doesn't fully negate fire damage? You can't explain Marigold way like this, or king, or luffy for that matter. Or other dfs that dont have anything to do with fire like fucking mochi. Or the sword techniques. They all utilize the same kind of power, some use outside heat starters, others who are better don't. Sanji developt that shit on his own, not because of genes. If he was the only one to show these types of moves, then maybe you'd have a theory. But he isn't, so it's logical to conclude that they are related and come from the same kind of technique. His heart burning hotter than his flames is just pre timeskip haki talk. Sabji and the trio all awakened haki pre timeskip without knowing it and sanji developed his first diable jambe from that, which needed an outside heat source to get started, which was the spin. Now he just says flame on like jhonny fuckibg storm because he has trained it.