This is not very helpful. When you are telling someone, "You are not role-playing," by what measure is he or she not "playing the role"? How is it that you are in a better position to say how "this person would act"?
Well, I personally try to avoid the phrase, "You are not role-playing," in part because of what you note right here. But also because I find it to not be a useful piece of constructive criticism.
But, unless you are playing a Vulcan tactical genius, if I ask you why your character did a thing, and you always point to the rules and note how it was the most effective choice, then you're probably not playing the role, you're playing the rules.
Ultimately, the question isn't whether or not what you are doing is role-playing, but how you might make your role-playing better.
There are very basic questions as to what you mean by "the role" and "this person".
There's a reason why we refer to our units in RPGs as "characters" - role-playing is a lot like acting, or writing fiction. In those realms, there's also basic question about what role is being played, and who the characters are. A large point of the exercise is to explore those questions. That doesn't mean we can't talk about methods to improve acting or writing.
The "theatrical" interpretation seems to be the prevalent one here.
Yep. Largely, I think, because the game is a group activity, and theatrics are the basic way to communicate your playing of your role to other people, such that they can interact with it.