3 ideas
Magic in the story works like an ecosystem: it depends on the old landscape and the practices built around it. When railways, telegraph lines and machinery replace that landscape, the power leaks away. No one has to fight it. It is starved out by a new order that simply makes it irrelevant.
Yan's essence survives the loss of her original substance. She realizes it in engineered brass, because identity lies in the pattern of desire and ability, not in the material it runs on. The spirit does not refuse the machine. It adapts to it, and the tools of the colonizer become a way to restore what the colonizer destroyed.
The empire takes more than land and resources. Her reclaiming of that mechanized body as a weapon turns the instrument of extraction into an instrument of retribution.