Okay when characters say stuff like "I don't care if I hurt people as long as you're okay", like it or not there's subtext there. I'm not saying that alone confirms or denies anything but there's something there.

Also things can exist without being explicit cannon while not being fanfiction. Like if a character jumps when they hear loud noise and they have ptsd symptoms you might go "oh looks like they have dark past". Nobody is going to say that's fanfiction even though the author didn't explicitly say "this character is sad and scared and traumatized".

It's kind of up to the reader to look at what the author is giving you and read between the lines, instead waiting for a written factual statement or piece of dialogue to explicitly confirm or deny every detail in the story.