It is not misleading to use the singular names of illustrious historical figures, especially in the case of Julius Caesar, who is one of the most famous in history, and while an argument could be made that it can potentially "mislead" the less informed, it is irrelevant. If you don't even know of the Caesar, from whom all subsequent Caesars derive their name, then you shouldn't be injecting your opinion into the discussion anyway. It is understandable, but one could not help but lose respect for your historical credibility. The equivalent would be, in the context of scientific (although the context, in both of these cases, is almost insignificant) history, for somebody to complain about referring to Albert Einstein as simply "Einstein," or Sir Isaac Newton as simply "Newton."
You're correct in your claim that Augustus (i.e. Gaius Julius Caesar Augustus, whose previous name was Octavian) was most likely an INTJ himself; Julius Caesar was as well.
That being said, it is perfectly acceptable to refer to "Caesar," instead of Julius Caesar, in a casual discussion, where I was simply referring to the Caesar for the sake of simplicity.
I think the situation stands like so: you attempted to be a, for lack of a better term, smart-ass, were pretentious and as a result wrong, and were put in your place because of it.
You're also correct in that you are derailing the thread, and are also shaming the INTJ type, and should now depart.