Hmm, OK, but I'm not sure Apple Engineering's "works as intended" response should apply to System Events.
If this makes sense for the Finder:
The fact that Finder is displaying the eject icon is based on NSURLVolumeIsInternalKey being false (the drive is external), not due to NSURLVolumeIsRemovableKey or NSURLVolumeIsEjectableKey.
I don't see why it shouldn't also make sense for System Events (and Image Events), since all three have the same description in their dictionaries:
Can the media be ejected (floppies, CD's, and so on)?
The only practical difference I can see is that Finder has an eject verb whereas the other two don't, but it still makes little sense to me that we have two properties with the same name, same description, and belonging to the same class of object that return different values.
Since that radar isn't about System Events in particular but the underlying API, I'm going to BR it against System Events in 10.13 (at least that way I know it'll get seen).