I think you've sort of crossed the alt-right definition with the general internet definition.
In short, shitposting is just making dumb jokes, often entirely nonsensical. To use an example from a Twitter account I follow: "sucking cockingly" or "deep in gAy balls mode rn.: might not recover.."
The alt-right posts bigoted messages and says "I'm just shitposting" or "I'm just joking" as a way to seem acceptable to the general populous while dogwhistling to alt-right groups and people, and slowly radicalizing those who come into contact with it.