Dual boot (old model) MacBookPro9,2 running new macOS (APFS) without rEFInd
If macOS does not appear as an option from the Mac Startup Manager (i.e. holding down the alt key at startup), then you have something wrong with the macOS installation. This problem would be independent the installation of Grub and Debian.
You have not specified which version of macOS is installed. With the latest versions of macOS, the bless command can not set a default startup unless SIP is disabled. So, most users execute the bless command from macOS Recovery. You have not indicated if the internal macOS Recovery is bootable on your Mac. If not, then your model Mac can access macOS Recovery over the internet.
It would be helpful if you could boot to macOS Recovery and post the output from the command diskutil list. Of course, you should choose a version of macOS Recovery that can support APFS.
Most versions of Linux can be booted from the Mac Startup Manager by default or by copying certain Grub files to the EFI/BOOT folder in an EFI partition. Here the boot file needs to be renamed to BOOTX64.EFI. The problem with copying files is when Grub is updated, then most likely the copied files will not be updated. This is why many users prefer to install rEFInd to the EFI/BOOT folder instead.