O NOES!!11111!1!!!one!! (or some other geeky response)
Fortunately there was no corruption. This post is simply to pass on the tip of what to do if you happen to see this, instead of following the knee-jerk reaction of immediately uninstalling and reinstalling.
The issue here is misinformation. It's not necessarily corrupted at all - the installer displays this message if it cannot find the installation database file. When you installed RAD Studio the location of the installer file is written to the registry within this key:
HKEY_CURRENT_USER\SOFTWARE\Embarcadero\BDS\21.0\CatalogRepository
The value ServicePath in that key points to a .gof file, in my case F:\radstudio_10_4_esd_104203a.gof. The thing is, I installed from a .iso file, which I mounted to a drive (F:) at the time and had since unmounted, ∴ that path no longer reached anything. Unfortunately the installer responded inappropriately: misinformation.
I gather this issue will be fixed for the 10.5 installer, but hopefully this saves someone the trouble of an uninstall and reinstall at some point.