At the time I found it mildly frustrating because of the stupid casing on top of the pointlessly annoying pedantic distinction destined to cause confusion. As if there were another "OS-X" of any significance.
Wikipedia has references about it where Apple themselves even back then used either pronunciation, though. Also a reference that says while it stands for 10 it also references the X commonly found in many Unix things because of the Unix roots of Mac OS X.
It was clear that the OS wasn't thought of as a separate thing in the early days. The latest one looks like a typo.
2011: https://web.archive.org/web/20111130104524/http://www.apple....
2012: https://web.archive.org/web/20111130104524/http://www.apple....
Interestingly Lion was called "Mac OS X Lion" upon release[0], but then their website[1] called it "OS X Lion" predominantly.
[0] https://www.apple.com/newsroom/2011/07/20Mac-OS-X-Lion-Avail...
[1] https://web.archive.org/web/20110705041542/http://www.apple....
Actual 2012 link: https://web.archive.org/web/20121207053101/http://www.apple....
It's like how "Mac" is still short for "Macintosh", even though Apple basically never uses the full name anymore.