Only the same images between different object-types are not deduplicated.
Not quite - images can be deduplicated between different Sprite object types. Only images between Sprites and non-Sprites (e.g. Tiled Background) are not deduplicated. From the tutorial you linked to:
[quote:2f1x2y4j]images are not deduplicated between Sprite objects and other objects, since Sprite object images are spritesheeted and other objects are not. For example, a Sprite and a Tiled Background with identical images will still export two images, but two Sprites or two Tiled Backgrounds with identical images will only export one image. Usually these are both edge cases and it should not be a serious problem.
Also image deduplication is a pretty quick part of the export process, even with large projects - image recompression takes far longer, and you can skip that.