Is a corpse more flexible than a living person?
According to the science in this article: healthunify.com/why-cant-everybody-cant-do-a-split/, not really. Flexibility such as splits are only possible by lengthening the skeletal muscle fibers (in the case of splits, this includes the hamstring and iliopsoas). Which means that splits aren't solely dependent on how your CNS functions.
Furthermore, with death eventually comes rigor mortis where due to the body's chemical changes after death (lack of ATP production etc... https://biologydictionary.net/rigor-mortis/), the body becomes rigid which makes it even more difficult for a dead person to do splits than a live one without seriously damaging the dead person's body/muscles.