To create an UEFI-bootable USB installer for ESXi 6.5 (or newer) on a Mac, follow the steps below.

First, you'll need an USB flash drive. Any recent drive will be large enough (you'll need at least 512MB). Also, download the latest ESXi installer ISO from My VMWare. You probably need a My VMWare account.

With the prereqs in place:

Insert the USB flash drive in the Mac and start up Disk Utility.

Erase the USB flash drive and choose to format it:Partition map = MBR

Filesystem = FAT32

Give it a descriptive name of your choosing.

After erasing the drive, we still have to mark the one partition on it as "active". Disk Utility does not support this and needs to be done using Termi  ...