Without a human operator assessing risk, what's to stop the drone from accidentally flying into someone and cutting them up with its blades?
I suppose the answer is that the owner needs to be responsible with its usage. The example footage seemed fairly reasonable use-cases (no bystanders).
In addition, 'cheap' blades have sharp edges due to the manufacturing process. Some drone owners recommend sanding them to get rid of those, to minimize injuries.
Anything else is not harmless. Even a tiny quadcopter can easily cut your skin, because the plastic is hard and sharp. Think of it as one of those plastic picnic knives, except not that sharp --- BUT rotating very, very fast.
I've been building and flying quads for years now and Anything over the tiny indoor ones commands a LOT of respect from me due to experience.
The idea of these things autonomously following you is cool but a HUGE liability.
Unless they have some major obstacle avoidance on board they you basically could never safely use this around other people and honestly you shouldn't even use it around yourself.
Telling people the blades are harmless is very irresponsible.
I personally haven't seen this technology in DIY multirotor parts, but I know that the AR.Drone has what they call "cut-out detection". They use it to prevent damage to the craft rather than humans, but the technology could be tuned for this as well.
If a $60 device has it I bet the multi-hundred and thousand dollar devices will not have trouble.