The the CMY (Cyan Magenta and Yellow) theory is only theory it is not that RBY (Red Blue and Yellow) are not primary colors. Rather than reexplaining, I will suggest people look at https://color-wheel-artist.com/primary-colors/ where it is explained quite well
The biggest reason people started making this mistake is the advent of printers. They started using CMY ink cartridges. This explains why people use CMY and think it is different from the RBY primary colors.
"Where will you drive your own picket stake? Where will you choose to make your stand? Give me a threshold, a specific point at which you will finally stop running, at which you will finally fight back." (Derrick Jensen)