When learning React and Flux I was confronted with the following narrative:
The Stanford - Developing iOS 9 Apps with Swift - 2. Applying MVC has a good explanation of what MVC is, my take on it is similar:
If I want to be even more brief then I can just say: MVC is about decoupling logic from presentation.
MVC is about decoupling logic from presentation, Flux is about decoupling logic from presentation a specific way, which makes Flux based on MVC.
When Facebook developed their application they were using an MVC structure that was wrong for the problem that they were trying to solve. When they realised this, they just switched to a different MVC structure and decided to call it Flux.
I can understand that a lot of developers associate MVC with specific implementations of MVC and Facebook wanted to differentiate themselves from that, it's a shame that they made a bunch of factually wrong statements along the way.
See also: http://voidcanvas.com/flux-vs-mvc/