There's a lot of chip packaging in Taiwan, Malaysia, and maybe Singapore so these A16s are probably racking up frequent flier miles. Probably not China though.
I think "for assembly" here means iPhone assembly, ie. the final SoC will be sent to China to assemble the iPhone. I don't think GP is referring to packaging.
The dies themselves are "assembled" - cut from the wafer, bonded to the wires (or solder bumps) that carry signals to the rest of the system, and packaged for physical protection and thermal management.
In recent times, multi-chiplet architecture has added its own layer of complexity to that process.
In the future they will probably be packaged in the US: https://www.apple.com/newsroom/2023/11/apple-announces-expan...