Correct. But Windows is not changing the BIOS boot order. It will however change your EFI to make itself the default boot again, even if you configured it to use a separate ESP on a completely separate disk and the boot menu residing there. That’s by design as you can access your efi settings from a running system via software, while your chances to change your BIOS settings from outside the BIOS are slim to non-existent.

Also every BIOS not decades old can indead run with a hybrid setup of GPT formated disk with a small partiton to replace the MBR functionality.