You may ask the same question to the following "short" list of Chinese citizens who switched their citizenship and play for foreign countries. Why would it be an issue when the receiving end is China. Note the bold red texted people are known to have been top players in China's National teams just like Hannah Miller.
Table tennis player:
小山智丽, Japan
倪夏莲, Luxembourg
洪剑芳, Australia
严新, Australia
胡海明, Australia
韩莹, Germany
单晓娜, Germany
刘佳, Austria
刘园, Austria
李倩, Poland
刘娟, USA
王慧静, USA
周鑫, USA
冯天薇, Singapore
于梦雨, Singapore
林叶, Singapore
田志希, South Korea
崔孝珠, South Korea
寇磊, Ukraine
......
This is a incomplete list of former Chinese citizens playing for foreign countries in 2020 Tokyo Olympic game. There are totally more than 20. They have absolutely no foreign heritage (relationship by blood).
Weight lifter
赵常玲, aka Зүлфия Салахарқызы Чиншанло, Kazakhstan
I will stop digging here.