The difference is that they keep the last 5 keys in the rotationKeys property of the did document instead of deleting them. Which means people can still validate the past, but only the last 5 keys. 

In any case, they don't resign the past with a new key. So, at somepoint your old data disappears. If your past key has leaked, you should not keep it in the rotation set otherwise other people can create valid events under your key's name.