The UNMAP VAAI primitive, and space reclamation in general, is only used
when the datastore is thin-provisioned on the storage-side. The idea is
for the ESXi host and storage array to agree on what blocks can be
reclaimed and which are in use. When you thin-provision VMDKs on
thick-provisioned datastores, only the ESXi hosts need to be aware of
what space is available.
So in short, you can either make the current LUN thick provisioned on the
storage side (then choose thick or thin VMDKs), or you could create a new
thin-on-thin datastore and Storage vMotion VMs to the new datastore, after
which you blow away the old datastore.