I wonder if the latency difference compared to the aya neo is because of Wine/Proton. I've played through Journey on Wine and couldn't help but notice the camera input being 2-3 frames late.
As Steve explained in the video, the Steam Deck's input-to-photon latency is lower than the Aya Neo's simply because it has a much more powerful GPU.
The only real way to test if there's any extra input-to-photon latency would be to compare the exact same game on a Steam Deck running SteamOS to a Steam Deck running Windows.