The “Export As Image” item is grayed out because it is currently only implemented for maps. To create an image from all tiles in your tileset, you could create a new map, select all tiles in the tileset and put them on the map, and then save that map as image.
Of course, this will only work properly when all tiles in your tileset are the same size. This is often the case, but it doesn’t need to be. So when I would implement this for tilesets directly something smart may need to happen.