(they made the ever-so-popular error of indicating that snakes "unhinge" their jaws - and they were not talking about the mandibular symphasis).
As Skitt's Law would have it, it's _symphysis_...But to repeat your point, the mandibular symphysis of snakes isn't a synovial joint (with smooth cartilage surfaces and a joint capsule and all that stuff). It's just two bones and a ligament that can stretch, something synovial joints are practically incapable of.