I believe that has always been the law hasn't it? I think several people were tried and hanged after Lincoln's assassination who were not involved in the actual killing, but because they knew about the plan beforehand.
I don't think just walking out of a meeting and verbally withdrawing from the group legally clears a person of guilt for failing to report an assassination conspiracy.
Depending upon the timing of the withdrawal, it might IF one could convince a jury that one did not, and should not have, believed that such a conspiracy would actually be carried out. If one could convince the jury that one withdrew from the group because, e.g. "those people just kept dreaming up all sorts of useless crazy ideas nobody in their right mind would actually carry out, while I'd been hoping for a group that would plan legitimate actions which a sane person could actually do. I had no idea they were actually serious about those plans--I thought they were just a bunch of harmless but useless kooks."
Mrs Surrat was hanged for the Lincoln assassination and she had no knowledge of the plan.