Not all peripheral nerves are myelinated
The C fiber mentioned in the article (from Wikipedia) is unmyelinated, so SFPN cannot be a demyelinating disease. Still, I think GBS could damage C fiber nerves. There is evidence that damage to a nerve causes nearby undamaged C fibers to have increased spontaneous activity (reference to a [I]Nature Neuroscience[/I] article found in the Wikipedia article on C fibers). My guess is that the inflammatory response is probably the main mechanism for most people. If GBS also leads to axonal damage, as several variants do, I can see no reason why the damage would not be to all types of axons, C fibers included.
Yet another reason why figuring out peripheral neuropathy is so difficult.