Tried iPhone, Samsung, Huawei, Xiaomi. Volume buttons work in iOS but burst mode is too slow compare to native iPhone burst. That’s why the question appeared - why is is slow and why some devices don’t have even slow option.
I can't speak for LR Android, as I work on the iOS team. And all I can say about iOS is the LR in-app camera uses the public APIs that are available to developers outside of Apple. Still, I will ask the engineers if any new improvement here is now possible.