Figured it out. I downloaded the MS store version instead of the Steam version. Duh.
Note the GSX version is exactly the same, and no files are duplicated, which is one of the many benefits of installing using only Symbolic links to the Addon Manager folder\MSFS, instead of placing actual files in the Community folder.
So, in case you have both the MS Store and the Steam versions of MSFS installed at the same time, when you select the one to use from the Installer, you are just saying to the installer which of the two separate Community folders you want it to operate on, but the GSX Pro installation (and other sceneries as well) will only be one, just linked to the simulator version the installer is currently set to operate on.
Another advantage is, since there's only one copy of all FSDT products, when you update them, they'll be updated in both versions of the sim.