This isn't "music theory" (although almost everyone thinks it is). This is "music jargon and notation systems". A theory is something that explains an observed phenomenon. This jargon and notation stuff doesn't explain music, it just guides you through the jargon and notation. I have a music degree and learned all this stuff and a lot more. This presentation is an excellent , high-quality overview of this non-theory…
Isn't that how language works though? If everyone calls this music theory aside from a small minority of people, then it is music theory. "Music Theory" as a term is a study on it's own, it's not "the theory of music" and you can't really use the terms interchangeably like that.