This is according to developer Steve Troughton-Smith (via AppleInsider) who discovered the evidence in the recently leaked iOS source code. As a brief history, Apple’s latest Apple TV was released in 2017 where it is powered by the A10X Fusion chipset which powered Apple’s 2017 iPads and was also a variant of the A10 Fusion that was part of the iPhone 7 and 7 Plus in 2016.
Speaking of unreleased: there's an A9X variant of the Apple TV that never shipped, according to iBoot
— Steve Troughton-Smith (@stroughtonsmith) February 8, 2018
Prior to that, the fourth-generation Apple TV was released in 2015 where it was powered by the A8 chipset, which apparently could potentially been powered by the more powerful A9X, according to the leaked source code. This is because 2015 is when iOS 9 was released, which is what the source code is based on.
2015 is also when the iPhone 6s and 6s Plus were announced which were powered by the A9, with the A9X variant shortly after. That being said it is unclear why Apple might have abandoned the Apple TV with the A9X, but then again companies do test out variations of their products from time to time, so we suppose this doesn’t really seem like a stretch of the imagination.