Obsidian is an electron app which means it is essentially Chrome under the hood. You’ll need to handle CORS like you would in any other client-side cross domain request just as the error indicates.
You could try throwing no-cors on the fetch request. Or Google handling cors with fetch.
If you want to go the quick-n-dirty route lookup ‘cors anywhere’. I wouldn’t ship a plugin with cors anywhere though as it’s liable to break.
Setting up a server of Glitch is pretty fast and easy tbh. Just use the express template and move your cross-browser fetching into the glitch app.