Which is appropriate since OS 9 immediately preceded it and was indeed the meaning of it. They rebranded it to “macOS” with Big Sur I think, or later. In the early days which I referred to it absolutely was “Mac OS X”.
Felt that more than a little too. I try to remember time didn’t start the day I turned 18, and likewise have at least learned all manner of things out of date or lesser used before my time (os2/warp comes to mind) out of just general interest. Even dos qualifies here.
MacOS. X means ten in Roman numbers.