Ah ha - I had been configuring syslinux to serve out pxelinux.0 only and not pxelinux.efi. Could they have just thrown that one liner in to one of the slides?

http://www.syslinux.org/wiki/index.php?title=PXELINUX#UEFI

edit: on 2nd thought I need to try this out to verify it end to end. Maybe there some other step.

Or maybe this is yet another reason to embrace AWS. I get to declutter the UEFI/BIOS topic out of my mind.