Gardener's multiple signature hack allows you to choose multile signatures.... or no signature from a pull down.
Changes are extensive though.. the way signatures are handled in threads there's not an easy way to on and off them on a post by post basis.
In classic... the signature was stored in the post, so it was easy to switch on and off. But if an image breaks or a link in a sig changes.... you have to edit every post.
In threads... the sig is stored in the user table, and output when the threads are displayed. You change the sig, and all old posts update with the latest sig. Thus no outdated signatures.
That's where the difference is.