That is scary! Our DDs are the same age and I totally could see her doing that because she over confident on the stairs. I had one of my girls flip backwards off a rocking chair on my porch, go head first over it down 4 stone steps onto concrete below and was right there & couldn't get to her in time a couple months ago. It was terrifying but she too was fine (just a bruise on her hip which she landed on).
My blood pressure went up just reading that, and I had flashbacks of when DS1 fell down 3 steps. Longest 2 seconds of my life!
I'm so glad she is OK. Kids are crazy resilient, their bodies are so much different! They also don't tense up when stuff like that happens, which is often what causes adults to get hurt.