[diffeomorphism]

> this sounds like it'd not be more than a stylus sensor behind the display (as usual)

And that would be a wrong guess. Eink readers with wacom styluses are not particularly new (e.g. Hanvon, onyx etc.). The remarkable 1's "claim to fame" was a greatly improved latency compared to these, i.e. to be much better than what you describe.

[namibj]

I tried one. And the screen did not seem any faster than the "EInk Carta" one in e.g. the Tolino Epos readers. I assume they fixed the issue with software being in the way of latency, and seem to make extensive use of partial refresh. And, yes, they did seem to integrate on a much lower level than any other recent consumer HID->screen drawing pipeline.

Back in the days of the C64, it was normal to have sub-frame average latency with +-0.5 frames jitter, due to low-level control that prevented artifacts when drawing directly to the framebuffer.