Unless the physics package is a gun type assembly (Little Boy), detonation is unlikely, maybe a “fizzle” measured in tons. The laser need not vaporize the target, it could do even as little as damage the re-entry vehicle such that it breaks up when hitting the atmosphere. No explosion, but you do get a few kilograms of fissile dispersed in the atmosphere.
I guess I wasn’t clear. I didn’t mean to say that a nuke warhead would denotate when it hit the ground; just that it wasn’t going to be destroyed by the laser. As you said, our designs probably won’t give a nuclear yield, but who knows about Korean/Iranian technology?