That not how I know and use the term (and I've been professionally involved in R&D for software and hardware for 35 years); it doesn't necessarily mean the product doesn't and never existed, and is only a marketing poy, only that it's not available to the public after repeated announcements & delays. That was the original historical use of the term (see the
Wikipedia article).
I didn't claim (nor do I think) that Onyx or the other vendors set out to mislead the market intentionally; however, given that even existing devices (Icarus 8, Pyrus Maxi) were discontinued after months on the market it looks like the market for the 8" e-ink devices is small, so the companies are being very cautious about committing mass production to it.
As to why the market is small, I'm not sure, but after doing research this morning, I found out that an equivalent 8" LCD 4:3 Android tablet from a reputable vendor is < $150 in the US (and there are no-name store brands for as little as $100(!)), maybe it's not surprising.