I agree with all of you! However, while I think those bendy backs are only given by nature, Erifilly is not a natural long-lines/superflexy girl, IMO. But she has incredible technique. I know girls who can fold in half and bend their back completely, but have a hard time even holding a hollow. Then there are girls who are so powerful, can do standing front tucks etc, but will lose the ability to do a split if they don't stretch regularly. And then there are the ones who have a little bit of both and can be trained in either direction, wich are definitely the ones that fit best for the sport, IMO.
But I think this is what makes the sport so interesting. And why the 2008 Olympics were my favourite: Shawn and Nastia, so different, but both so amazing in their own way. I love Simone, but I have to say I also really love performances that are very artistry and feminine, not only pure tumbling power. But the variety is what keeps the sport interesting!
However, to come back to the original question, I think that the kind of flexibility that Erifilly shows in the video is not unhealthy, as every movement is very controlled. I think this is what every young gymnast should aim for. But only taking it as a perfect example, not working towards it for hours, risking injury for something that is not necessary.
I tried it myself yesterday, and I am sure that the secret lies first in active flexibility (being able to hold a front split completely against gravity. Like lying on the floor, lifting one leg all the way up to your nose. And second a very flexible shoulder that is able to balance the arched body, weight almost completely over the hands.
Thanks to everyone for their input!