Well, do they? The operating system can provide microcode updates to a running CPU. Can the operating system patch the PCU, too?
When I look at a "BIOS update" it usually seems to include UEFI, peripheral option ROMs, ME updates, and microcode. So if the PCU is getting patched I would think of it as a BIOS update. I think the ergonomics will be indistinguishable for end users.
