That is correct, each LUN is an extent. Prior to VMFS5 there was a 2 TB limit, but since then I think it is easier administratively if each datastore is one extent/LUN.
Storage I/O Control does not support datastores with multiple extents.
You never want to have more than a 1:1 ratio of LUN per VMFS datastore. Even spanning a single VMFS datastore across multiple extents is to be avoided.