The ES6 arrow functions are not a problem, the issue is that you are mixing them with ES6 Class notation. Class methods don’t need arrow functions — see https://eloquentjavascript.net/06_object.html#h_hPv1gHC33s for more information about ES6 Classes.
Unfortunately I’m not familiar enough with React to know how to get its JSX syntax working in JSFiddle… it seems to show errors now, and I have no idea how to fix that. Here’s a version without React, which doesn’t seem to be necessary to make this work: https://jsfiddle.net/donmccurdy/xb4f7nvu/
The MeshSurfaceSampler class is only available as an ES module, so you will need to import it rather than using global scripts. The example above shows that too.
Finally, the requests to your FBX files are failing. If you open the browser developers tools you’ll see CORS errors about that. I think the issue is that githack (or anything else, really) can’t access a private github repository.