looking at the present map of Gansu, I can see the

geography. ZGL was northwest of Jie Ting. In this case, ZGL perhaps was facing someone or he was just preparing for the next attack phase. So he send this protege Ma Su to defend the choke point of Jie Ting from Zhang He's attack. ZhangHe used 50,000 troops. That translator above had misinterpret Mao. Mao meant that ZGL should have been guarding Jie Ting against Zhang He in the first place and not send Ma Su there. That translator say ZGL should reinforce Jie Ting from the North, but wasn't north of Jie Ting was Wei's land. ZGL thought that Ma Su could hold off Zhang He if he did he was told to do. So no matter what, ZGL was too late to stop Zhang He. If ZGL attacks from the north, then all Zhang He had to do was hold ZGL off at Jie Ting using ZGL's original tactic and call for reinforcement to attack ZGL's back while he send forces to cut off ZGL's supply and may pillage into Shu to decrease ZGL's troop morale. I don't think Mao would be that stupid to suggest ZGL attack Jie Ting from the north.

I didn't see anywhere saying ZGL was fighting someone when sending Ma Su. I read that all resistance at that mountain was finished.