That depends on what one means by "booting into BIOS mode". CSM is a shim than makes UEFI firmware look like BIOS to the OS (or, well, bootloader) and to x86 co

That depends on what one means by "booting into BIOS mode". CSM is a shim than makes UEFI firmware look like BIOS to the OS (or, well, bootloader) and to x86 code in various option ROMs, so you are always booting into UEFI. Really booting without UEFI being involved in the process would require completely different firmware image.