That doesn't seem to fit the common usage. The salt accompanies the message when the message should be treated with some amount of skepticism or something like

That doesn't seem to fit the common usage. The salt accompanies the message when the message should be treated with some amount of skepticism or something like that. If no skepticism is needed, no one mentions taking salt with it. The amount of salt mentioned then increases with the amount of of skepticism needed. So, if much more suspicion/skepticism is needed, we see that people mention much more salt. The salt is like an antidote for the questionable sources, not just a flavoring.