I solved this in a different way without having to worry about whether there are hidden files or not in the EFI partition. I have 2 test systems of HS one on a hdd with hfs+ and the other on a SSD with APFS. Here is what I do from within a terminal session. dd if=/dev/disk0s1 of=/disks1s1, when complete I copy the proper apfs.efi driver into the drivers64UEFI folder of the EFI partition of the APFS formatted hdd.
For me this works every time without having to think too much and also without having to spend hours troubleshooting. Caveat when using this method: both EFI partitions need to be unmounted, and also make sure that the medium described by the "if=----" statement is a working EFI boot partition of your hfs+ based HS system. Doing it the wrong way will leave you in limbo not being able to even boot from your hfs+ formatted drive anymore. Of cause the medium described by the "of=----" statement needs to be your EFI partition of your APFS formatted HS macOS.
Greets