OCI has no idea what is going on inside a thick LUN (as provisioned on the storage), or a thick VMDK (as provisioned in VMware).
This is because neither storage arrays nor VMware know what is going on inside the thick object.
As a general rule, you would need an in-guest / in-host agent to retrieve that data. You get a feel for this when you think about one of the rare cases where OCI does know what is going on with a thick LUN - if you create a Vmware datastore on a thick LUN, from a storage array perspective, OCI only know the (allocated) capacity of the LUN. However, vSphere can tell us what the utilization is of the datastore, which is insight into a thick LUN. In this case, vSphere is acting like a host agent because vSphere intimately knows what is going on inside the ESXi hosts.