If you are just using it as a standard ESXi with VMs running on it (including the Witness Appliance VM) then it is not part of the vSAN cluster and thus you can use whatever licensing you want.
If you are using it as a Physical witness:
"Licensing If using a physical host as a vSAN Witness Host, it must be licensed with a valid vSphere license. This does not require the same licensed edition as the vSAN Cluster it is supporting."
Thank you for the clarification, saves us a few pennies.