Movies without racially diverse casts

[THGhost]

I got into an argument with someone the other day. We were discussing the fact that Mad Max: Fury Road has a distinct lack of "non-white" characters.
They claim that any film that doesn't have a racially diverse cast, it is the filmmakers behind it that are responsible for being racist.
This person, from what I could tell, was a female African-American feminist, so their opinion may be a little bias. But their words did shock me slightly.

If a film doesn't have a racially diverse cast, everyone involved with it is instantly classed as a racist? I simply cannot understand this statement.

They also say that any film set in a post-apocalyptic future should abide by the laws of today, i.e. racial equality and therefore cast multiple "non-white" actors in its leading roles.
Did I read this right? We don't live in a post-apocalyptic future. Films set in them are allowed to make up their own laws and filmmakers are allowed to cast whoever they want, regardless of their race.
Besides, there are at least 3 "non-white" actors in Mad Max: Fury Road. Is that not enough for them? I pointed out to them that if the entire cast weren't white or if it was equal, people would still want it to be different. They completely disagreed with me on that. Lunacy.

Please don't any of you intemperate what I'm saying the wrong way. I am in no way a racist.