You only have an exteme overpressure where you have an atmosphere, In space you only have thermal and radiation to work with, you'd only have force if you could get a warhead into the surface.
True: impact is required. Buried is best, but an impact far enough out may be “good enough”
A nuclear blast contains lots of high speed matter blasting out of the center as well as EM energy. A properly built bomb would have a shell of matter to achieve its purpose, converting and conveying mass impact (KE) to the target.