The thing about Nimona (the movie) is, as you begin to watch, you think the knights have been fighting off monsters, like, regularly. That if not the last few generations, than at least a 200, 400 years ago, right? They've built the wall and they've made the Mega Destructive Canons, and an 1000 years worth of (possibly a tad inbred) Knights trained to fight monsters, and you think, well, they must have USED them
And as the movie goes on you realise, people haven't just never seen a monster in recent memory, the Kingdom has never seen a monster At All.
Gloreth was a child, she turned on her friend because her parents told her to. Nimona left after Gloreth turned her sword on her. She "defeated" the monster.
Gloreth's home was destroyed by the foolishness and hatred of the adults around her, but she was raised to believe it was her friend's doing. Her memory of the event was probably twisted by this narrative, until she remembered it as the little girl turning into a fire breathing monster (Nimona never actually turns into anything that looks like that dragon image in the scroll)
Then, raised to believe she'd been the hero, told over and over again that her positive memories of Nimona were lies, Gloreth grew up hating "monsters"
But she never actually fought them. Her knights never fought them.
Because the monsters never existed