Github Pages site not detecting index.html

It got fixed automatically. I just had to wait for a while for the settings to take effect.

Pushing a second commit fixed this for me.

Seeing other answers where changes fix this, my guess is that you need to trigger a few of deployments to get it to work.

Every push will trigger a new deployment. You can track deployments at https://github.com/username/username.github.io/deployments.