Peace find I none after having slain my grand-father, and that foremost of men, Karna, who never fled from the field of battle.
Here are some incidents where Karna fled
Shrouded with showers of shafts shot from Bhimasena's bow, Karna once more turned his back upon Bhimasena. Beholding the Suta's son turn back and flying away on foot, covered all over with the arrows of Kunti's son, Duryodhana said, 'Go ye quickly from all sides towards the car of Radha's son.' http://sacred-texts.com/hin/m07/m07132.htm
Then, thrown by the Rakshasas who had become more powerful in consequence of the night, there began to fall on the field of battle a thick shower of stones. And a ceaseless shower of iron wheels and Bhundis and darts and lances and spears and Sataghnis and axes also fell there. Beholding that fierce and awful battle, the kings, thy sons, and Karna, also exceedingly pained, fled away. http://sacred-texts.com/hin/m07/m07152.htm
And at this, Karna's vision was obscured and his senses left him. And regaining consciousness, he felt a great pain, and leaving the combat fled in a northernly direction. And at this, the mighty car-warrior Arjuna and Uttara, both began to address him contumely.'"http://www.sacred-texts.com/hin/m04/m04060.htm
Their are several other instances. Like Abhimanyu, Drupada, Ghatotkaca, Gandharvas, Suvraja, Satyaki etc. So Yudhishthir's statement makes no sense.