That doesn't seem true, sorry.
A witness authorized by CGC (store I guess, or whatever) can crack a yellow label and add a signature too, and have it resubmitted too, and it doesn't have to be at the CGC booth. I've had it done quite often.
So what I don't understand, is why can't CGC give some of the pressing companies out there with the same authority? CCS is not very good compared to the top pressing and cleaning guys out there, so why not authorize them as witnesses so people can get their SS books done for a potential higher grade?