So if he chooses to kill Rob Tenzin, Yongzheng won't choose to keep him as a soldier, which will weaken his power and even kill him, so he let Rob Tenzin go for his future safety.
And at that time, Yongzheng couldn't continue to provide supplies to this 100,000-strong army, because the daily consumption of this 100,000-strong army was huge. So at that time, Wu Sidao had already talked about the current situation of Qing Dynasty in Nian Gengyao. If there is no decisive battle, then Nian Gengyao will bring down the Qing Dynasty alive, his own army will be disbanded, and his status will naturally decline. So nature is not what he wants to see.
Moreover, in order to prevent Yongzheng from having doubts about himself, Rob can't immediately find the trace of hiding Dandan and break it. He can only point out the way for him through Wu Sidao as a reason to send troops. At that time, everyone knew that Wu Sidao had never been to the northwest. So it was in this case that Nian Gengyao sent troops to crusade, but he didn't choose to kill Rob Tenzin, but let him go quietly, because he knew that Rob Tenzin would want to make a comeback as long as he was alive, so his position would naturally not be affected by this matter.
He asked Rob to hide Tenzin, so that Yong Zhengdi could continue to trust himself, his position could be stabilized, and he could continue to be a general. Make sure you are successful all your life.