Based on Diamond Taira's backstory, this is a Spiritual Prequel. Meaning it's not eveythign fits into this story's timeline but is essentially what happened.
It's a new story for a new audience so having ABS have gotten her Cutie Mark for beating the shit out of Die seems a little out of place and I didn't want to explain why Scootaloo wasn't there either so I ignored those facts and said 'alternate timeline!'
Either way, enjoy the new story!