It is easier with systemd-boot because the kernels are stored on the EFI partition where sbctl is able to automatically find them. When the kernels are stored on a separate partition (as they are with Garuda Linux), you need to identify the kernels and explicitly sign them.
That’s where this command comes in to play:
Alternately, you can sign the kernels one by one with sbctl sign /path/to/kernel.
After signing the kernels, when you run sbctl list-files it should show both the bootable .efi files on the ESP (which sbctl is able to find automatically), and the kernels in /boot (which you just explicitly signed).
No, that is not an issue. Probably you are looking at something like this:
Available Boot Loaders on ESP:
ESP: /boot/efi (/dev/disk/by-partuuid/cae9a5da-1c50-4f20-99f2-63f46ec756c9)
File: └─/EFI/BOOT/bootx64.efi
That is the fallback UEFI bootloader path. Your UEFI firmware will attempt to use that bootloader if no other bootable options are configured. It is not related to your GRUB installation.