Knives do not immediately incapacitate – often, a person who has been stabbed is able to continue fighting for long enough to successfully retaliate, even if they die soon after. This makes knives both a poor choice for self defence, and a good choice for murder. A horrific way to fight since ancient times.
An important point to learn from this video: striking is an effective response to a knife attack, assuming that you are a capable striker. Many people tend to focus on the knife and fight to control it, not realising that this is as good a time as any to punch the face.
Most people tend to overestimate their fighting ability. This man is off the mark further than most when he is surrounded by 5 youths (including the one operating the camera). He initiates the violence (flicks a cigarette at one and pushes another) but takes no appropriate action – allows himself to be surrounded, does not move, does not position himself correctly or attempt to isolate and incapacitate his opponents. He appears to falsely assume these youths will cower and back off.