It was never really meant to be "OS 10", despite what the 'correct way' to say it was. Pure marketing.
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"?
I think he meant that it was not supposed to be "Mac Oh Es Ten Ten Nine Two", but rather "Mac Oh Es Ten Nine Two" when referring to Mac OS X 10.9.2. Do you think people said "Mac Oh Es Nine Nine One", or "Mac Oh Es Nine One" for Mac OS 9.1?
I think you write (and say as /ten/) "X" as the major version if you're also using the non-numeric minor version: "Mac OS X Snow Leopard"; but you omit it when using the numeric minor version so as to also use the Arabic major version: "Mac OS 10.6". You say /ten/ either way and never twice.
People seem to quite commonly write "Mac OS X 10.6", which duplicates the 10. News articles and Wikipedia included. I don't remember what the machines themselves reported, though. I don't have anything pre-Sierra anymore.
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"?