Originally the Niro EV was supposed to get NCM 811's, a more advanced battery chemistry from SK Innovation over the NCM 622 used in Kona. SK reportedly had delays in 2018 but were supposed to go into production of the 811's in 2019... did they make it to the 2019 vehicles? This might explain why we are seeing such good range for the pack. We already speculate the actual pack is larger than the 64kW since it has been reported DCFC charging current doesn't taper as it gets to 100%, so there is probably a buffer in the top-end of charge. I am hereby starting the conspiracy theory now that the cars today actually have 811's (hence the small number of vehicles built & sold due to supply of 811's) and its a secret to be unveiled as a larger capacity pack in next model-year vehicles. https://pushevs.com/2018/05/09/most-advanced-battery-cells/
AC charging (max 7.5 kW) does not taper/throttle, but DC fast charging does. There seems to be general agreement that the actual pack size is closer to 70 kW...
It appears your guess on the batteries is correct. https://electricrevs.com/2018/12/20/exclusive-details-on-hyundais-new-battery-thermal-management-design/ According to the article, the Niro does have a different (and more advanced) battery from SK Innovation than the Kona.
That article is from 2018, and the original plan was for Niro to have the newer battery, but then there were delays. https://electricrevs.com/2019/05/31/report-sk-innovation-to-begin-making-nmc-811-cells-in-q3-2019/
So, the Norwegian 2020 Kona (as of March 2020) will get increased range according to this article. The range is increased from 449 kilometers to 484 kilometers. Might the NCM811's be landing in the 2020 vehicles with greater range in USA? Hmmm...
The Niro battery max capacity is 67 kWh. Soft capped at 64kWh. https://ev-database.org/car/1260/Kia-e-Niro-64-kWh I love the conspiracy theory. I wondered if myself if the Niro has the NCM 811. I also wondered if the feud between SK Innovation and LG Chem caused delays in NCM811 battery shipments in 2018. However news says the feud affected VW battery supply chain, not Kia's. http://en.thelec.kr/news/articleView.html?idxno=493 In the next 5 years it is likely the next NCM811 batteries for Hyundai and Kia are made by SK Innovation. https://www.autoblog.com/2019/12/21/hyundai-kia-sk-innovation-ev-batteries/
Might be just as well the Niro has the 622 batteries... "Reducing the ratio of Cobalt can change the cell characteristics in undesirable ways that need to be compensated for with other changes such as using new particle coatings or electrolyte additives. Changes to cell design require intensive testing."
No doubt the intensive testing will take place, however the proof is in the pudding. Until they are used under normal usage conditions for years we won't really know if the manufacturing technique is producing a good product.
