The distribution needs to do almost nothing to support either EFI or the legacy BIOS or any other booting method.
That is the job of a bootloader package, not of the Linux distribution. For example I am using syslinux as the bootloader for all my computers, while grub is another example of a frequently used bootloader (but which seems to be excessively complex in comparison with syslinux).
I assume that by supporting only EFI Fedora means that they will remove all bootloaders from their installation image, so that the Linux kernel will be launched by its EFI stub.
I do not know about other bootloaders, but an installed syslinux package occupies only a few megabytes, maybe 10 megabytes at most, so deleting it cannot provide much space for anything else.
I cannot see how deleting a bootloader package may be claimed to be a significant simplification for the maintenance effort of the Fedora distribution.