Ryou is the new fad in town, but tell me again why ryou is better than hardening. Vergo masters hardening for a special reason, to be more indestructible. Pica saw Zoro using ryou but was confident it was not going to measure up against his hardening defence. ONly problem is Pica's CoA didnt match up, or FBH spread it too thin for Zoro to have stronger haki flow over haki defence. Vergo survives the SAD by help of this. No flowing haki ain't helping you on that front.
Again, it's very reasonable to asssume DD used barrier based on Sanji's internal heating ability. DD could clearly sense Sanji was a powerful opponent as he looked a bit shook and nervous, yet he just used Koka for Luffy and looked cool as a moose. You saying DD look shook from Sanji yet used no Koka, implying he doesn't have barrier and used no haki to block? I mean that on its face makes little sense. It's all things considered reasonable to assume he put up a barrier haki defence on the spot to guard himself against Sanji. The way the fire is dispersed makes little sense if it isn't barrier.