Okay, actually watching the video helped ;o).
The Kindle is using a different waveform mode (my guess would be a regal one, probably REAGLD). I don't recall anyone ever doing that before, because REAGLD used to not be worth it over GC16. IIRC, the Kindle reader used to do the same thing as for text content, which is REAGL until a flash (which is what we do in KOReader, too).
I haven't checked that in a while, though, and a Libra 2 might behave differently than a Sage/Elipsa, and/or differently than a PW5 on that front

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EDIT: My Elipsa behaves similarly (i.e., no switch to black/mini flash on white gutters), as long as I don't flash on every page. (Because that enforces GC16 instead of REAGL).
TL;DR: Double-check that you're not requesting a full refresh every page in KOReader (Gear > Screen > E-ink settings > Full refresh rate).