Hi,
It would appear that on the EX9253 all of the interfaces are channelized from 40G to 10G by default. After a 'show interfaces terse' all of the interfaces show up as xe-0/0/0:0 and so on. I have been totally unable to find any documentation regarding channelization specific to the EX9200s. I have only really found this document which only mentions EX4600s and QFX10Ks. The disable-auto-speed-detection doesn't work as it isn't supported on the EX9253.
I did find that I could set the speed specific to the port or pic but that didn't work even after a system reboot.