Understood. However, there was some discussion online (I don't remember where) about the Niro battery being closer to 68-70KWH, with 64 usable. The remaining capacity was reserved such that the battery, even if showing 0% to the driver wasn't actually at 0% state of charge. Similarly, if the driver sees 100% state of charge, the battery wasn't actually charged to 100%. There was that additional capacity reserved at both sides to automatically protect the battery from repeated 100% to 0% to 100% cycles. This is the same thing the Chevy did. In a Gen 2 Volt, the battery's capacity was confirmed to be 16.8KWH, but the usable capacity was limited to about 14.4KWH.
So, it's interesting that Kia would put that in the manual when there seems to be built-in measures to prevent that 100%/0%/100% cycle already, and I don't think any of their warranty language specifies that the owner should follow the 20% to 80% rule to be covered by the warranty.
From personal experience, I charged my 2016 Volt just about every night for 3 1/3 years so far, and experienced zero range/capacity loss in that time. However I had terrible capacity loss in the 2013 Leaf over 3 years I chalk up the difference to the very different approaches to battery management by Chevy and Nissan.