I would argue that monocultures exist because of it's relation to other existing cultures. See:

https://en.wikipedia.org/wiki/Monoculturalism

That said, you are right and monoculturalism is not the right term. It is a political term at best and a charged cultural term in today's racial and cultural environment. It's too late to change the title. Sadly, the rest of my post doesn't mention the term, but the responses don't seem to address anything but the title.

I think it's a hypothetical, not an oxymoron. A monoculture could exist, but if it did, you'd just call it "culture," which maybe is what you're getting at: The moment it is realized, the qualifier is redundant. I don't know if an English word for such a thing exists.