With its position in the Army Table, it could be mistaken as a mount - most other mounts are listed after the General, Heroes and Wizards in their respective Tables.
But, having just read the Special Rule for it, it looks like it isn't a mount and it's actually a machine.
The GS on top is probably artistic license from the sculptor. Other Generals can be mounted on flying critters so aren't truly "earth-bound" compared to any Skaven characters who don't get access to any mount.