Kropotkin is wrong about "communism" in the Middle Ages. If I were feeling kind, I'd say he has misinterpreted a couple of particularly violent examples as representative of the wider political atmosphere. If I were feeling unkind, I'd say he's trying to make the evidence fit his argument rather than the other way around, which is bad practise.

I've written about Kropotkin and his misunderstanding of medieval communes in this old answer.

If you want to read a bit more about why rulers would create communes see this answer by u/WelfOnTheShelf

The closest the Middle Ages gets to what Kropotkin describes is the Peasant's Revolt of 1381, in which one of its leaders of one of the rebel groups, Wat Tyler, called for the destruction of nobility as a class and common ownership of all land. Today we'd call Wat Tyler a Communist, but he was in the minority even among the more radical rebels - most just wanted lower taxes and a few crown officials dismissed - and parallels between the Peasant's Revolt and modern communist movements are dodgy, for reasons you can read in this answer.

Another issue with Kropotkin's treatment of the Middle Ages is the assumption that peasants were largely incapable of successful political expression other than through violence. As I've recently written about, peasants could be rather involved in the running of the medieval state and did, at rare times, genuinely feel that the nobility worked for their interests.