That's the fun part!
Factoid kind of became a factoid (I know there's an actual etymological term for this but I can't remember it)
I believe its original definition was an assumption thats circulated enough its assumed as fact.
ie many "wives tales"
But now the definition you gave has been assumed so much it has become fact!
See? Fun!
It was originally something that seemed like a fact, but wasn't, similar to you would describe something that looks like a human, but isn't, as humanoid.
I guess people thought factoids they read were true, and assumed it just was a weird true fact.
Combining sound and food stimuli is classic pavlov conditioning. Cats learn very quickly if they make any sound that a human will react in a way, ie scratching a door for territory or meowing for food.