I was able to test a few configurations at home using the 8bitdo pro 2 controller. I have confirmed that a wired 8bitdo pro 2 in X-input mode does get picked up by the Sequoia operating system. Unfortunately the game does not pickup these inputs.
I ended up testing the windows executable on my M1 pro via a Whiskey bottle. Unexpectedly, Grunn running on Apple silicon via Whiskey accepts X-input AND D-input out of the box (tested with an 8bitdo controller running D mode wirelessly, and X mode wired).
So if you are looking to play Grunn with a controller on Apple silicon, it seems like you will need to run the windows version via Whiskey for now. Anyone reading this that doesn't know what Whiskey is, just google "Whiskey macos".