The supply line was known as the lifeline because if it got destroyed, more than a 100,000 soldiers at the front line would get plunged into starvation.
No matter how brave the soldiers were or how great their commander's military expertise was, once they ran out of food, it would be a terrifying event that would inevitably lead to their defeat in war.
When P5092 came to Mt. Zuoyun at the beginning, he was not actually planning to disrupt the barbarians' supply line. After all, the barbarians would definitely guard it heavily, and that was exactly how it was. In order to protect their supply line, the expeditionary army surrounded Mt. Zuoyun with 70,000 troops.
Therefore, P5092 only wanted to force the expeditionary army to come here and trade their lives for the position. He did not think his strategy would pose any threat to the barbarians' supply line.