I have followed the guide too, installed Clover and everything works fine. Make sure when you install Clover that it's for the right setup. I had to change the setting for UEFI only (click on customise tab) because the default settings was for legacy (BIOS). It means it won't create the EFI partition with the necessary files needed for booting in UEFI mode.
I wanted to add that when I first installed High Sierra and Clover the EFI partition was formatted in FAT16 and not FAT32. I had to format it to FAT32 manually (dual boot with linux made it easy) and then copied the usb EFI folder content on it. I realised that something was wrong when Clover Configurator wouldn't mount the EFI partition (SSD not HDD).