Unless I'm missing something, the post is plain wrong in some parts. You can't POST to a Cross-Site API because the browser will send a CORS preflight first before sending the real request. The only way around that are iirc form submits, for that you need csrf protection.

Also the CORS proxy statement is wrong if I don't misunderstand their point. They don't break security because they are obviously not the cookie domain. They're the proxy domain so the browser will never send cookies to it.

Anyways, don't trust the post or me. Just read https://owasp.org/ for web security advice.