It absolutely is a mirage. It's like those Ferrari kit cars where you take a Ford or whatever car frame and remove the outer shell and put a Ferrari shell on top of it. It's not a Ferrari, it only looks like a Ferrari.

The browser engine is the majority part of the browser, everything else around it is window dressing. So when you install Firefox on iOS, you are getting Safari with a thin wrapper around it. You are not getting the Firefox rendering engine, which is the most important part of a web browser.