Turns out the game is being made on Unreal Engine 5:
Aside from Fortnite which transitioned across a while back, I believe this may be the first Unreal Engine 5 title on Switch.
Previews are also out and seem to be quite positive:
https://www.ign.com/articles/lego-horizon-adventures-preview-lets-do-a-silly-one
https://www.gamespot.com/articles/lego-horizon-adventures-isnt-your-typical-lego-game/1100-6524180/