Why does rendering on a PC take 10 seconds compared to 0.25 seconds in-camera?

He is talking about rendering the RAW data to get the embedded JPEG.  It doesn't get there by magic, the camera's hardware/firmware renders it.

LR on the other hand first previews the embedded JPEG that the camera rendered, and then renders the RAW data to produce its own version of a JPEG.  LR's  preview is fast only because it need not render the RAW data to get a preview.  The camera does not have that advantage, as there is no JPEG available at all until  the camera renders the RAW data to produce the embedded JPEG.