Hi Chris -
Apparently GarageBand doesn't recognize the Aliases and thinks the sounds are not installed. [Actually Logic/GB continually re-downloading library content is a common issue, though there are no definitive fixes.] Some people recommend creating Symbolic Links instead of Aliases, claiming that they work better [they should in theory, that's how Logic does it with their new option for re-locating library content] -- a search for "symbolic link garageband" or "symbolic link logic" should turn up some recommendations.
If I remember correctly the GB essential library content is only around 2GB [I think the full content d/l is around 14 GB] -- unless your internal HD is really tiny (i.e. 128GB) it might be easier just to leave the library where the installer puts it -- that's what I ended up doing after using Aliases for years..