You cannot turn this buffer off, probably because writing the stream of bytes into the eInk screen directly is a very nich use case.
Is it? IMHO that's one of the biggest advantages of EPDs like eInk --- you don't need a framebuffer, since the display itself is a "write-only memory" and only needs lines shifted into it.
On the other hand, I suspect EPD companies try to keep the low-level details (like driving waveforms) secret because that's a large part of their IP. They want to be able to sell controllers for them too.
The most unfortunate part is that line-based JPEG decoding and even downscaling is a solved problem, but the bloat of today's software and comparatively limitless computing resources seems to have lead us to forget how to write software efficiently.