One for negation experts

Northern Ireland expert Denis Murray, on BBC, talking with an interviewer about the Queen’s historic meeting with Martin McGuinness.

Interviewer: Would you say that until recently something like this would have been unthinkable?

Murray: I’d say more than that, until two years ago it was not even thinkable.

It’s pretty clear that Murray felt he was adding information. But why does “not even thinkable” mean more than “unthinkable”.

I believe he took the interviewer’s “unthinkable” to mean “you can (indeed must) think it, but you really should not do it”, and augmented by using “thinkable” to imply that “nobody would even have thought of it, or have needed to judge that it was a bad idea”