You are correct. You cannot upgrade an existing HIPS 8.0 installation to Patch 2 using the full installer. It will fail. You must use the incremental upgrade package to upgrade HIPS 8.0 RTW/P1 to Patch 2.
HIP80LMLRP2.Zip - Full installer; used to upgrade from a previous HIPS version (e.g., HIPS 7.0), or for a fresh installation of HIPS 8.0 (no HIPS installed); pushed via an ePO Deployment task.
HIP80P2.Zip - Patch upgrade package, used to upgrade from HIPS 8.0 RTW or Patch 1; pushed via an ePO Update task.