It still throws the 404 erro and now I am not sure whether this is an issue with the repo or with netlify. There is an index.php file though, so I am grateful for any hint why the error persists.
The short answer (from a quick test building locally) is this template won’t work.
A longer answer…
As mentioned under the Prerequisites this template uses node.js (only) to build the front end, while using PHP for the backend. Netlify does not support PHP at runtime, only at build time. This is why if you navigate to https://thirsty-jackson-90a256.netlify.app/index.php the intended index file for this project index.php is downloaded by the browser rather than having the server render the content.