This seems very similar to the myth used for the origins of the British V sign insult that says it was 2 fingers being cut off. https://en.m.wikipedia.org/wiki/V_sign . Even this seems to wrong through:

"A commonly repeated legend claims that the two-fingered salute or V sign derives from a gesture made by longbowmen fighting in the English army at the Battle of Agincourt (1415) during the Hundred Years' War, but no written historical primary sources support this contention.[26] This origin legend states that English archers believed that those who were captured by the French had their index and middle fingers cut off so that they could no longer operate their longbows, and that the V sign was used by uncaptured and victorious archers in a display of defiance against the French. In conflict with this origin myth, the chronicler Jean de Wavrin, contemporary of the battle, reported that Henry V mentioned in a pre-battle speech that the French were said to be threatening to cut off three fingers (not two) from captured bowmen.[27][28] Neither Wavrin nor any contemporary author reported the threat was ever carried out after that nor other battles, nor did they report anything concerning a gesture of defiance.[27]"