…So, I checked into my BIOS and found out that apparently the motherboard supports both the Legacy and UEFI BIOS…
So the previous answer - “It’s Legacy BIOS” - was not correct. It’s UEFI one.
A true Legacy BIOS doesn’t even mention UEFI. Legacy BIOS knows nothing about UEFI. It may not be “converted” into UEFI, in no way.
Only UEFI bios may work in both [UEFI] and [Legacy] boot modes. The boot mode is set by entering in bios setup utility during boot. The bios setup utility is usually entered by pressing F2 if you boot in [Legacy] mode, or by using OS- and computer’s make- and model-specific software tools if you boot in [UEFI] mode.
…I tried installing again after deleting that partition, but to no avail. Got the EFI in the checklist there…
I made a second installation to test your case. This time on a computer with UEFI. As UEFI is needed only by Win8+ crap, which is not used here, UEFI bios on this computer is set to boot in [Legacy] mode.
The rest remains the same as during the first installation (on true Legacy BIOS computer). The stable antergos-2015.07.01-x86_64.iso release is used. The disk has no /boot/efi partition.
Cnchi 0.8.151 has no EFI checkbox even on UEFI computer:
I’m out of ideas, at this point.