Angelo Dundee first came to prominence because Carmen Basilio liked the way he wrapped his hands- not because of his 'genius.' He was picked to 'train' Ali precisely because he would not train him: after the experiment with Archie Moore training Ali, it was decided to put him in the hands of somebody that wouldn't try to teach him anything. Leonard was the same way- he was finished product when he got to Dundee and was there because Dundee wouldn't/couldn't mess with his style. He is considered a great trainer because he once said "You're losing it son" to a fighter that already knew that and was fighting accordingly.
Or, in short form, he's been senile since the mid-1950s.