I find this to be a self-serving claim. If it were true, then the claim that contact with American indigenous people was an eye opener — as the authors claim — becomes credible.
But is it true?
Before the discovery of the New World, there were various peasant disturbances. [See: Popular revolts in late-medieval Europe] What were peasants grumbling about? Since most peasants were illiterate, we have to go by what they did and by reports of what they said they did. But we can do better by following the sequence of events following the Black Death (1346-1353), in which about a third of the European population died.
From an economic perspective of supply and demand, the demand for peasants increased while the supply of peasants decreased. The result was a wage inflation for peasants.
Consequently in England King Edward III in 1349 passed the Ordinance of Labourers, and Parliament in 1351 passed the Statue of Labourers. These laws made work compulsory, and set controls on wages and the price of goods.
In 1381, the discontent with the prevailing state of affairs burst forth as the English Peasants’ Revolt.
We may take the sermon of the priest John Ball (preaching at Blackheath in 1381) as expressing the sentiments of the peasants.
“When Adam delved and Eve span,[a] Who was then the gentleman? From the beginning all men by nature were created alike, and our bondage or servitude came in by the unjust oppression of naughty[b] men. For if God would have had any bondmen from the beginning, He would have appointed who should be bond, and who free. And therefore I exhort you to consider that now the time is come, appointed to us by God, in which ye may (if ye will) cast off the yoke of bondage, and recover liberty.” [a. Delved meaning dug the fields, and span meaning spun fabric (or flax). b. “Naughty” then having a stronger sense of “evil, malicious”.]
And here we have an expression of a grievance against inequality, which contradicts the claim of Graeber and Wengrow that no one in the Middle Ages would or could make sense of the problem of inequality.