I am member of a character fandom that exists for several years at this moment. There were several discussions to it, but after some time most of them were defined. One of these discussions was about giving a certain name to a character who doesn't have one; the fandom has soon come with a fitting name, had some time being happy over it, and that was that. In fact, I think of myself as the one who finished this discussion by articulating "this headcanon is becoming generally accepted", but I might exaggerate my merit in this.   
So, this discussion was basically over, and after some time, this headcanon was used where appropriate (i.e. this name is used in fanfiction almost exclusively when there is need in one), but not mentioned specifically; it was considered a general knowledge for fandom members.

That is, until one day I came across a post that said something like *"Hey fandom, \[name\] is the best name for \[character\]!"*  I don't claim these words to be exact, but I can't provide actual wording because I'm unable to see the actual post. 

Because of my "merit" described earlier I was annoyed to see this rise up again but didn't want to argue with poster publicly, so I DM'd them, saying *"The \[character\] fandom has somewhat agreed on this, like, year and a half ago, but thanks anyway."*   
Their reply was *"Nice! I wasn't in the fandom back then, I have not seen one post calling \[character\] by that name, I have no idea who you are and considering the first thing you ever say to me is a sarcastic snark remark on a dumb \[site\] post i dont wanna know you! Bye!"*   
Then they immediately blocked me so I wasn't able to resolve this conflict.

So, in the end, what was it? Does my knowledge of fandom give me the right to tell people when their ideas are outtated? Did my ego push me into arrogance and desire to control what others are allowed to speak about? Or was it all a mere misunderstanding, making the newcomer respond aggressively when they saw aggression in my message?