WHQL has nothing to do with secure boot at all. Just thing about non-WHQL graphics drivers, they do not stop any device to use secure boot. WHQL is just a gimmick used by Microsoft to earn money and give IHVs the right to use the "certified for windows" sticker. Secure boot just needs a proper platform key, nothing more, it a UEFI specification not Microsoft. WHQL is only needed to install drivers on a clean Windows installation (thet are not required on an installation upgrade), otherwise generic driver or no-driver at all will be used. After installation you only need proper signed drivers and WHQL is not needed. It's simply a Windows (clean) installation thing. Secure boot will continue to work anyway.