I'm using this all the time myself. There is no login to YouTube required and it supports adding subscriptions and doing everything important you can do on YouTube.
Switched to PipePipe because Tubular is not updated as quickly as Tubular, the dev said it's not on the top of their list. Also PipePipe has some more features such as playing the video faster while you leaveea finger on the screen.
I use NewPipe on regular Android but read somewhere it wasn't recommended to be used on Android TV because it wasn't designed for it. If you vouch for it working fine then maybe I'll try it.
It's using the YouTube API, so that makes sense. The developer likely wants to keep the app focused and reduce the amount of feature creep, hence reusing YouTube's standard subscription and list systems rather than building their own. That's also what the majority of users expect - if someone subscribes to a YouTube channel on one device, they'd expect to be subscribed to it on all devices.