And as bootloader_id also defines the directory containing Manjaro’s efi files; which also differentiates it from other distributions potentially installed (multi-boot); living with it works for me.
Either that, or changing the wording to something like;
"Warning: GRUB bootloader at $(dirname $efi_source_file) was updated. If you are not using this instance of GRUB by default, please check your EFI boot priorities!"