It's not exceptionally revolutionary technology. It's a technique that has been described before.

Yes, if-and-only-if you had the source file before and after information had been embedded, then absolutely you can tell some encrypted data must have been added(but not necessarily what it was).

However, as a third party(say an oppressive government) looking at maybe images published from IPs within your country and trying to determine if any contain encrypted messages, it is conceivably impossible because you don't have the original file. Since compression already introduces a level of noise, if your encrypted message doesn't introduce more noise than is present then a third party can't distinguish an innocuous image with normal artifacts from compression versus those that have artifacts resulting from embedding encrypted information.

If I generate semi original images such as a meme and embed data in those, then third parties don't have any original files to generate hashes of for comparison against. This is where you're misunderstanding how these techniques are applied.