The video you linked to is a basically a brawl in the street in the 20th century with non armed combat. Apple to Oranges. I also agree with you about martial arts movie where a single guy takes on 20 people, not very realistic. (however have a look at how a guy can hold off multiple opponents in non armed combat
https://www.youtube.com/watch?v=X8_zWBQXZj4)
Here we are talking about a fully trained knight in his peak condition with really good armor and sword. The bandits/peasants in the video are not very well protected and don't seem to take advantage of their numbers due to inexperience/fear factor. Also in the video he does fight 3 or 4 people at once and they do try and make him fall to the ground, but again he is trained and knows how to keep his distance and stay upright and he also takes damage.
You could argue that it might be not 100% realistic, but I think it does show the effectiveness of knights. I mean think about it, if a knight could be taken down by a bunch of peasants, why did invest so much time and money on knights and armour.
Here is the video review by Skallagrim
https://www.youtube.com/watch?v=x80hOR42szg