I wonder why they still write "OS X". Intentional or just due to somebody's lack of attention to detail?
It was often compacted to MacOS for various reasons, like brevity, keyword searchability and tagging.
"macOS", for all its initial lowercase goofiness is at least understood by most users.
If it was really meant to be "OS 10", then they wouldn't have versions like "OS X 10.6", they'd just have "OS X.6" or "OS X.VI". How many people do you hear saying "OS ten ten point six"?