If you want to use the Calamares installer, it is Btrfs or nothing. If you don’t use Btrfs, a lot of the Garuda-specific tools will not work. There is not a lot of sense building support into the installer for other file systems, only to have people complain when everything is broken because they chose ext4 or whatever.
Of course you can! 
An easy way to accomplish this would be to just install Arch Linux with whatever filesystem and partitioning setup you want, then add the Garuda repo afterward and pull in the rest of the packages you need. We have some folks that do this in order to use Garuda Linux on ext4 as well.