Yeah, headcanon is definite fanfiction when it comes to works with one author and have a beginning, middle and end.
This (DC and Marvel) is pretty much a free for all at this point. They arguably just publish fanfiction now, it just ranges from being great to average to embarrassing.
So when things are changed, they just change into something else that itself will inevitably be changed somewhere down the line. And we know this at this point. The main behind the curtain is completely visible. If canon is only based around the fickle whims of the company controlling it, canon means nothing.
If they have that view of headcanon, I think they're talking about something else than what we are.
Canon can be as small as using a character or mentioning a previous plotline. That exists and does mean something to the stories that are being told.
Headcanon is described on wiktionary.org as "Elements and interpretations of a fictional universe accepted by an individual fan, but not necessarily found within or supported by the official canon." I think that fully delusional view previously mentioned would come under this definition of Headcanon.
“Somewhere, in our darkest night, we made up the story of a man who will never let us down.”
- Grant Morrison on Superman
Head cannon is fine when defining characteristics and morals but when you stick to it too much then you miss the opportunity to make new stories to be honest, hence why new 52 felt like a breath of fresh air at the time.
“Somewhere, in our darkest night, we made up the story of a man who will never let us down.”
- Grant Morrison on Superman
I would say that I'm fine with headcanon provided the fan understands that it is THEIR opinion/interpretation and THEIR opinion/interpretation alone.
I remember here a long time ago as well as on the old DC Message Boards, there would be fans who would make statements such as "Clark cannot be a dominant personality/Superman needs to be the most powerful/Lex has to be caught or it makes Superman ineffectual", and would add that if anyone felt differently they weren't "true" Superman fans.
If someone is/was a pre-Crisis Superman fan, great! I love the pre-Crisis Superman. But to turn it around and say that if you aren't a pre-Crisis Superman fan then you're really not a fan of the character...yeah, I have a problem with that. I think too much headcanon can make a fan feel entitled to something he or she isn't entitled to.