There’s a few reasons I can think of. I’ve been hacking Android for years.
Netflix is extremely finnicky, it closely couple with DRM, and is very version specific. You can find a specific APK works but another doesn’t. If you’ve managed to release a platform which works, with a specific NetFlix version, you’d want to prevent auto updating via Play Store.
Android TV is a separate platform, different from Android, is less popular in quantity so less supported.
I tried looking up the specific product, Nebula II does not exist, there’s the Nebula Mars II ? To not be confused with the Nebula Capsule II.
I had the Capsule Max, a different product, and sideloaded and it all worked fine for me, but on other Android like systems, like a Android VM running ontop of Linux, only a specific NetFlix APK version would work. So with the imprecision of the product, the Mars II is Android 7.1, the Capsule II is Android TV.
Basically the fewest issues arise from buying the most popular products which get the most app love. I did try one of the Nebula products, and using this rule I avoided Android TV to avoid the least popular software.