I don’t know if this will work … in fact I would like to know if it does 
When you boot your system - hold down Shift key until you enter the grub menu.
In the grub menu you can both edit an entry E or you can use C to enter the console.
Press C and input fwsetup and press Enter
If this doesn’t work you can use the console with the solution from the topic you mention.
When you have gotten your firmware entries back - go into the firmware and disable EFI - then reboot with a Manjaro installer.
Then you can wipe the EFI partition and create another unformatted partition and flag it as bios-boot.
Then reinstall grub to the new unformatted partition.