Digital Foundry's Hands on video showcasing the XBOX version:
The game is still using the Unity Engine, but it is a newer build. My guess is that it's probably using the Vulkan API. But the developers are targeting 60FPS for the Switch version, which is also the base target console. The Xbox One X version is running at 60FPS and 4K resolution.
Playtonic are developing the game around the lowest common denominator, which is the Switch, and adding in more features for the other higher end platforms.
The game honestly looks really nice. A bit of a mix between classic Donkey Kong Country and Donkey Kong Country Returns. The game has a little big of elements like the Sonic series as well.