The difference between fake news, à la The Onion, and fake news that's propaganda is the propaganda has an agenda that isn't advertised.
Pieces in The Onion do have an overt political leaning, but the publication's primary function is comedy, not news.
A lot of these so-called fake news agencies may quietly label themselves as satire simply for legal reasons but make an effort to conceal this whenever convenient.
I'm not sure if you can categorically differentiate between the characteristics of an Onion piece full of hyperbole and some typical nonsense fake news piece, but the impression left in the readers is probably the differentiator: Fake news sites are trying to convince people that they're true, where actual parody and satire sites do not.
In the truest sense of the word ("that which is to propagated"), yes.
But for common usage, it's seen as less than honest. For example, consider "spreading awareness" and "spreading propaganda". Which seems benign and which sinister?
Mr Propaganda himself, Edward Bernays, had consumer-facing Propaganda renamed as Public Relations because people had a sour association with the word from the Great War, but openly stated that it was the same thing.