I am a Zoro fans, but even I can understand and accept DJ=friction.
Think of it this way:
1. Sanji is super fast. Don't you think he can just speedblitz a rotation to generate fire by friction?
2. It's about proficiency. Luffy needs to pump for G2 pre timeskip, yet he can do instantly post TS. Can't Sanji do the same? No need to spin so fast for a few seconds, he can just move his foot left and right so fast and hard, its enough to generate fire.
Now, the real problem is how Sanji maintains the fire. I always thought it was haki, especially since Oda said Sanji's heart burns hotter.