The drivers are signed, they just don't have WHQL certification. Microsoft requires that the drivers are signed with a special type of digital signature to load them on 64-bit versions of Windows Vista and newer - if the driver has this signature, it'll load, but Windows will display a prompt during normal install, unless the driver has passed WHQL certification, in which case it gets Microsoft's signature that allows it to load silently.
Booting in BIOS or UEFI mode doesn't affect this.