You might think it was a mistake for various reasons, but this isn't one of them. If it wasn't for UEFI it would have been coded for BIOS/MBR. The article literally mentions this.
In fact, this only works if combined with a Secure Boot disabling exploit. Secure boot (despite the MS key...thing) still would prevent the modified bootmgr.efi from booting.
The article works very hard to bury that bit.