"Fan fiction" and "headcanon" aren't one of the same. Fan fiction deals specifically with the writing of said materials in the form of a story. A headcanon is defined as "Elements and interpretations of a fictional universe accepted by an individual fan, but not found within or supported by the official canon."

"Headcanon" is about a specific idea or concept from one's mind upon reading a specific product's canon, backstory or otherwise. It's not official canon, but an idea from an individual's point of view that could be woven into the canon personally. It has nothing to do with "backstory," but headcanon can be used for specific backstories because the individual is blending an idea into the canon, whether it's in fan fiction, fan art, music, analysis, debate, or any other form of art. So, no, "headcanon" isn't used incorrectly.

The word canon means official. Head canon is like saying unofficial official. If you make up something about an official product, it is not canon. It is only canon if it is recognized by the owners of the property. As I said, we need a different term for it. But it is not canon, by definition.

how about we start calling it skulllore or ponyfluff

Except "headcanon" is defined as a concept or interpretation that the person's mind can accept as canon. Canon in his or her head/conscience/mind alone. That's been the definition for years, and claiming to need a new term now is completely foolish.

You have problems with slang, jargon, and idiom in general, don't you?

Canon meaning the official lore for a fictional work is a modern usage that would be considered a jargon term, first used in this meaning with reference to Sherlock Homes back in 1911.

It's 'jargon' because it's a technical term only used by 'insiders' (i.e. fans) Outside of fandoms the word 'canon' means a body of religious law.

I have a problem with improper use of language. If canon can mean something unofficial then it is meaningless. Anybody can make up a story for an existing franchise and it is canon? Tell me, then, what word do you use to describe something that IS officially licensed? Official canon? Then in a few years we will have the term "official head canon."

'Headcanon' has been around for about fifty years, first mentioned in a Star Trek fanzine if I remember correctly. The term is older than you are.

Improper use of language is nonsense. Words change meaning constantly in every living language, that's why it's a living language and not a dead one like Latin. Slang, colloquialisms, jargon, idiom, are all ways the language changes over time. Heck, if I actually switched to using original English, the language Beowulf was written in, you wouldn't even be able to recognize that it's English.

"When I use a word," Humpty Dumpty said in rather a scornful tone, "it means just what I choose it to mean -- neither more nor less."

"The question is," said Alice, "whether you can make words mean so many different things."

"The question is," said Humpty Dumpty, "which is to be master - - that's all."

- Through the Looking Glass