"I don't think being able to change the drive to AHCI mode would fix the problem."
One user Bownairo confirmed that changing the drive to AHCI mode did fix the problem by allowing the drive to be recognized in Linux, after manually flashing a modded BIOS by soldering chip programming hardware:
Props to that guy going above and beyond.
I wonder what it'll take to get Linux to see the drive in RAID mode.
New kernel drivers from Intel, most likely.
djpohly and the BIOS hackers deserve some credit too for their REing:
https://www.bios-mods.com/forum/Thread-Request-Lenovo-Yoga-9...
Conditional flag locking users out of the Advanced settings page (with the RAID settings) of the BIOS, as shown in disassembled code:
https://lnv.i.lithium.com/t5/image/serverpage/image-id/76843...
I am quite sure requiring such extreme measures shouldn't count as "support".