The name "x86_64" existed before AMD fully released their CPUs, and was latched onto by some of the open-source communities, so that specific one isn't exactly Intel's. But you're right about EM64T. :)
I, myself, had an AMD Opteron machine on which I ran an x86-64 build, with that name for the architecture, of SUSE Linux before Intel released their EM64T.
http://www.amd.com/us/press-releases/Pages/Press_Release_715...
Dated 8/10/2000.