I'm not sure it could be considered a 'mistake', because first of all in the context of the film, by revealing those things to Kay, he was revealing them to the audience as well.
And second of all there's no reason he would later regret it because at the time he claimed and believed that that was his family, not him.
Exactly. Legally, it would be hearsay and not admissable.
"Generosity. That was my first mistake." "Experience must be our only guide; reason may mislead us." "Instagram is Twitter for people who can't read."
