9216 is the typical supported mtu for ethernet. But earlier iterations of their network used devices whose jumbo mtu length was like 9100 bytes.

Aws uses A LOT of different overlay technologies which are implemented through encapsulation(aka overhead).

To enable cross region vpc connectivity, additional encryption and encapsulation can be layered on top of the existing encapsulation.

9001 bytes was the absolute max they could give to customers and still be able to do packet shenanigans.