I'll notify the devs about it... someone had the same issue (not sure if they were on Arc or not) and managed to play but not sure why. Sorry about this!
Once you uploaded your Godot web version on itch, you must check the "SharedArrayBuffer support — (Experimental) This may break parts of the page or your project. Only enable if you know you need it." box, under "Embed options" or "Frame Options"
