> FastCGI and HTTP are at two different levels. HTTP is for data transfer from, say, a browser and a server. FastCGI is for handling that data between the server and an application.

Not entirely correct. A reverse proxy can either speak HTTP, or a different protocol such as FastCGI with the application server. The article is talking about that communication.

They are not interchangeable for the browser-to-server communication, but they are for the server-to-application piece.