I think the most reasonable interpretation is that the law doesn’t currently recognize the undead as being people, let alone being the same person they were in life. It would need to be shown to be a reliable source of evidence, similar to any new technology that claims to offer insight into a case. A random judge might allow it, but it would be easy grounds for an appeal if it can’t be shown to do exactly what it claims to do.

DNA evidence was new once, but so was the polygraph. Only one of these is admissible, and for good reason.