I have seen this first hand (no pun intended) but on a lesser scale.
Worked as a bicycle mechanic for a year and a kid that worked at the shop caught the tip of his index finger on the front sprocket of a fixed gear track bike. That kind of bike does not use a freewheel back sprocket, so the weight/momentum of the pedals has a tendency of sucking rags and fingers in if your not careful.
It cut down to the bone so he had to go to the hospital, just a ton of stitches and a big bandage.
The older head mechanic who had been there for 20 years told me he has seen finger tips lopped clean off.
That looks much worse than what I saw that day.