I followed the provided tutorial, with the only difference being that I am using yarn instead of npx/npm. In my setup, running yarn deploy does not actually trigger the predeploy script, resulting in a confusing situation where I have made changes (locally) but the gh-pages branch does not get updated.
The obvious fixes are two: either manually run a yarn build before each yarn deploy or modify the script to do it itself (also, delete the predeploy script since it does not offer anything). I would suggest a small note on this (incredibly helpful) tutorial, so that other people do not get baffled like me :)