I found that the grub config for grsec kernel was incorrect and couldn't boot. Upon attempting to send a pull request I see that it is corrected in the git repo but is not reflected in the doc.
set root=(hd0)
set default="Alpine Linux"
set timeout=0
set root=(hd0) set default="Alpine Linux" set timeout=0
menuentry "Alpine Linux" {
linux /vmlinuz-hardened root=/dev/sdb modules=sd-mod,usb-storage,ext4 console=ttyS0 quiet
initrd /initramfs-hardened
}