Certainly there are fair reasons for hiding capabilities, and they seem quite adept at determining what is going into the sea or not, but in a war of words environment if it was a certainty as THAAD propenents seem to claim a single shootdown would be a huge PR blow for nk.

It's just not reliable enough to risk and as such is still a last resort measure.

Defending against a nuclear ICBM is not possible to do reliably. It's a sad reality.