They didn’t mention how large it is, nor why it isn’t likely to impact the Earth despite many close interactions.
Horseshoe orbits are pretty well understood. When I first saw the orbit, I though it looked something like a horseshoe orbit, and in fact, it was in a horseshoe orbit until around 1920, and in a couple of hundred years it will drop back into one.
Check out the wikipedia article on horseshoe orbits.