As I understand it, undecidability means that there exists no proofs or contradictions of a statement.
So if you've proved $X$ is undecidable then there are no contradictions to $X$, so $X$ always holds, so $X$ is true. Similarly though, if $X$ is undecidable then $\lnot X$ is undecidable. But again, this would mean $\lnot X$ is true which is a contradiction.