If it includes a marginally hidden container with an obvious structure this sense of "steganography" seems formally correct (secrecy without a secret key) but overly broad in practice.

Consider, for example, that you can apply "normal" steganography to the image data in the overlong PNG content (claiming, for example, that you just "hid" a porn picture at the bottom of a cute kitten while there's a secret message in the whole composite picture).