He isn't really alone, he has his horse 
Well, I was condering that said peasants A) had no armor, B ) were wielding pitchforks or less, and C) did not all of a sudden surround him, but had to use a LARGE radius in order for him to not notice, given speed advantage, and I was also assuming said knight was on an armored horse, had a Lance for charging and a longsword for in-fighting, and that most of the advantage came from large scary horse vs unarmored average joes. Therefore, most of the killing (70-150) would be while the peasants were encircling him, making the strangle-hold tighter and tighter.
Of course, your argument makes more sense if peasants had better weapons, basic armor, if the knight Was not riding a massive war-horse, and if peasants could magically teleport. However, most of the factors you weren't thinking about was because of giant warhorse vs unarmored peasants ....
but yea, there I clarified. Now, 10-20 knights could easily do what I was talking about ... im beginning to think that one lone knight might be a bit of a strech, unless he was a special unit/royalty/hero
I mean, 10-20 knights working together are about 100x more effective than a lone knight. But yea I guess I was thinking more-so of a hero-knight on horseback (and whenever I think of knight I normally think of horseback, unless someone specifically says heavy melee)
but yea, I never said he could kill more than 100 pieces of misguided meat over the course of a day ... but if you are completely outnumbered you get them to chase you ... and the fastest naturally break from the pack ... and you pick them off, especially since your on a horse which is many times faster than a beer-belly thug with a big stick.
That being said, if somehow magically you teleported the knight into a mob which already had him not only surrounded on the map, but also were all pressed up against him and such, it wouldn't be long before he was some-how badly wounded, and in the process of him and his horse dying, due to peasant density, most likely around 5-10 peasants would die. 3 from death throughs/kicking by horse, 2 before knight gets wounded, and 5 while fighting to the death. Possibly.
meanwhile 20 knights in full charge, tight formation, would trample about 50 unarmored peasants in a matter of seconds (if they got a running start, assuming they were poorly armed)