They use it like OSX and iOS which rightly have OS in their name: both are the new and original operating systems. Debian, Ubuntu rightly don't have OS in their own names. Elementary shouldn't too.
Poor choice of examples, mate. From the respective home pages:
1. "Ubuntu is the world’s favourite free operating system" - ubuntu.com
2. "Debian is a free operating system (OS) for your computer. An operating system is the set of basic programs and utilities that make your computer run." - debian.org
Then you have CentOS and their ilk. But that whole argument is irrelevant: take a step back and look at what you're arguing about. It is such a petty argument. I feel like I'm living in a crazy house where trivial things are important.
I finally stopped caring and accepted that "OS" has evolved for most end users to mean a user experience, an ecosystem of applications, a hardware support list, and a technical support strategy. For many developers, it's now the set of tools, services, and paradigms available for development along with an understanding of the number and types of users that can be reached by supporting it. These definitions span the classical distinctions between Operating System, distribution, and even sometimes application. In my line of work, I can communicate clearly with the majority of my customers by referring to things like Android and eOS as "OSes". It's what they expect and I'm okay with that.
Congrats on the launch of Luna.