I think you are confusing FSX/P3D products with MSFS products.
- FSX/P3D products have their own installer which can always be freely downloaded from the FSX/P3D products pages, so they don't need to be installed from the Universal Installer, this might change in the future, but is not really required now, so you can use the Universal Installer for Activation/Deactivation or Updates, but not to Install and FSX/P3D product.
- MSFS products don't have a separate installer, and are all installed using the Universal Installer.