Knowing what he's talking about would lead him to instantly recreate a recipe. In order to know what he's talking about, he has to be informed. How would he recognize something he's never experienced, and why haven't you found your pre timeskip example of the same yet?
You ask for proof of him learning or spending his time, but we didn't see much of any Straw Hat across the two year gap. They go right into the reunion from the start, so all we have are results. Not like we sat there and watched Zoro learn CoO or Chopper learn Kung Fu. Sanji was surrounded by cooking martial artists, pretty easy math. It must take a lot of work to go so far around the simplest idea and suggest that he went there and didn't get help with fighting while also not spending his time cooking.
You ask for proof of him learning or spending his time, but we didn't see much of any Straw Hat across the two year gap. They go right into the reunion from the start, so all we have are results. Not like we sat there and watched Zoro learn CoO or Chopper learn Kung Fu. Sanji was surrounded by cooking martial artists, pretty easy math. It must take a lot of work to go so far around the simplest idea and suggest that he went there and didn't get help with fighting while also not spending his time cooking.