What does this say about form criticism?

I was reading through Stephen Carlson's post about the Secret Mark session and I noticed this little section. Carlson was summarizing Helmut Koester's defense of Secret Mark (and Morton Smith):

First, it was most like the material unique to Mark (in Mark's disagreements against Matthew and Luke). Second, Secret Mark's account of the youth in the tomb is form-critically more primitive than John's Lazarus account, and Smith was no good at form criticism.


I wondered when I read this - if Secret Mark is ever put to rest as a hoax (and I think it is), what does this say about form criticism? If someone who is no good at form criticism can just happen to create something that, form critically, looks earlier than our canonical account in John, what does that say about form criticism's ability to date texts? If it could be shown definitively that S Mark is a hoax(and I think it has already), then I think the academy should re-evaluate how it uses form criticism to date texts.