This argument never really made sense to me. Anyone who is being deceptive is not going to tell people they're editing their comments.
It's the result of nothing more than a moral panic. There aren't roving bands of keyboard warriors rolling around making comments and then editing them to make others look stupid.
And even if there were, they could just include "edit: typo" and get away with it. Unless someone takes screenshots.
I think it says more about the community that everyone is expected to prove their innocence. Let's have a little faith in each other, we're better than that.