Onyx devices use Android as their OS right?
It's conceivable that IF dithering is a problem for us, that Android based devices, because it's somehow baked into the OS, have it even if they don't need it. This is 100% speculation. If the light was on we could blame PWM as a possibilty. The Kobo Forma has PWM. Also lights being set to "off" aren't always fully "off". The Kindle Paperwhites (at least older models) would still have the light on very dim even at setting "0". We know Dasung monitors dither.
Since Kindles use their own OS maybe it's dithering, they don't do it, and that's the difference in your case...or maybe your post is Amazon marketing poisoning the well since it's Prime Day 😉
I also know that even safe devices can seem like they are straining me if I am already triggered, or afraid they will. Sometimes it's hard to tell.