Her blast pushed back Chrysalis which is actually impressive considering Starswirl could only at most make Tirek flinch. Both times Starlight got knocked out she had her back turned and got caught off-guard, so you can't really say it counts. Not to mention the first KO gets negged by the fact that Starlight was back at full power and ready to fight in less than half a minute after she gets hit the second time.
Oh, and did I forget to mention she was exhausted when she got hit the second time?
Look at her. Freeing everyone clearly took a bit of magic out of her as evidenced by the bags under her eyes. So not only was she off-guard, but she wasn't even at full power when she got hit the second time.