Hopefully I remember this the next time I need a NGK part... I don't know what all goes into these types of decisions, but NGK is well known across many auto enthusiast communities as "the go to". Lots of internet history might be a little confusing.
NGK does not make aviation spark plugs, and while Rotax and experimental owners, have used them for years, NGK's position has always been that their plugs are not intended for aviation. Niterra on the other hand did / does produce aviation igniters for gas turbines. Essentially a sparkplug used to get a turbine running. I've not known them to produce spark plugs for reciprocating engines, but it sure wouldn't be a stretch so maybe they're getting in the game since NGK's were already quite popular with the non certified sector.