Thanks for testing!
'insmod ext4' would be wrong - GRUB's ext2 module handles all of ext2,
ext3, and ext4, and it doesn't have a separate ext4 module. I suspect
that this emitted error messages that you didn't notice, but worked
anyway because ext2 was built into the core image (at a guess).
Adding 'set root=' is indeed normal - grub-mkconfig generates both
traditional 'set root=' lines and modern 'search' lines. We keep the
former basically as insurance.