It’s not true that Jake LaMotta was never knocked down in 106 professional fights. Jake did hit the mat once during an abortive 10 rounder vs up and coming light heavyweight, Danny Nardico in December 1952.
Far from diminishing his reputation as one of the toughest men in boxing history, the aberration from a largely forgotten slugfest in Florida merely enhances it. A glimpse at the fight on YouTube shows Lamotta decked heavily against the ropes in the 7th round.